Use constants for organisation type

This reduces the chances of making a typo, because doing so will raise
an exception rather than fail silently.

app/main/forms.py

@@ -1375,14 +1375,14 @@ class BrandingOptionsEmail(StripWhitespaceForm):
     def get_available_choices(service):
 
         if (
-            service.organisation_type == 'central' and
+            service.organisation_type == Organisation.TYPE_CENTRAL and
             service.organisation.email_branding_id is None and
             service.email_branding_id is not None
         ):
             yield ('govuk', 'GOV.UK')
 
         if (
-            service.organisation_type == 'central' and
+            service.organisation_type == Organisation.TYPE_CENTRAL and
             service.organisation and
             service.organisation.email_branding_id is None and
             service.email_branding_name.lower() != 'GOV.UK and {}'.format(service.organisation.name).lower()
@@ -1390,15 +1390,21 @@ class BrandingOptionsEmail(StripWhitespaceForm):
             yield ('govuk_and_org', 'GOV.UK and {}'.format(service.organisation.name))
 
         if (
-            service.organisation_type in {'nhs_local', 'nhs_central', 'nhs_gp'} and
+            service.organisation_type in {
-            service.email_branding_name != 'NHS'
+                Organisation.TYPE_NHS_CENTRAL,
+                Organisation.TYPE_NHS_LOCAL,
+                Organisation.TYPE_NHS_GP,
+            } and service.email_branding_name != 'NHS'
         ):
             yield ('nhs', 'NHS')
 
         if (
             service.organisation and
-            service.organisation_type not in {'nhs_local', 'nhs_central', 'nhs_gp'} and
+            service.organisation_type not in {
-            (
+                Organisation.TYPE_NHS_LOCAL,
+                Organisation.TYPE_NHS_CENTRAL,
+                Organisation.TYPE_NHS_GP,
+            } and (
                 service.email_branding_id is None or
                 service.email_branding_id != service.organisation.email_branding_id
             )

app/main/views/agreement.py

@@ -6,6 +6,7 @@ from flask_login import current_user
 from app import current_service
 from app.main import main
 from app.main.forms import AcceptAgreementForm
+from app.models.organisation import Organisation
 from app.s3_client.s3_mou_client import get_mou
 from app.utils import user_has_permissions
 
@@ -14,11 +15,11 @@ from app.utils import user_has_permissions
 @user_has_permissions('manage_service')
 def service_agreement(service_id):
     if not current_service.organisation:
-        if current_service.organisation_type == 'nhs_gp':
+        if current_service.organisation_type == Organisation.TYPE_NHS_GP:
             return redirect(
                 url_for('main.add_organisation_from_gp_service', service_id=current_service.id)
             )
-        if current_service.organisation_type == 'nhs_local':
+        if current_service.organisation_type == Organisation.TYPE_NHS_LOCAL:
             return redirect(
                 url_for('main.add_organisation_from_nhs_local_service', service_id=current_service.id)
             )

app/main/views/organisations.py

@@ -69,7 +69,7 @@ def add_organisation():
 @main.route('/services/<uuid:service_id>/add-gp-organisation', methods=['GET', 'POST'])
 @user_has_permissions('manage_service')
 def add_organisation_from_gp_service(service_id):
-    if (not current_service.organisation_type == 'nhs_gp') or current_service.organisation:
+    if (not current_service.organisation_type == Organisation.TYPE_NHS_GP) or current_service.organisation:
         abort(403)
 
     form = AddGPOrganisationForm(service_name=current_service.name)
@@ -97,13 +97,13 @@ def add_organisation_from_gp_service(service_id):
 @main.route('/services/<uuid:service_id>/add-nhs-local-organisation', methods=['GET', 'POST'])
 @user_has_permissions('manage_service')
 def add_organisation_from_nhs_local_service(service_id):
-    if (not current_service.organisation_type == 'nhs_local') or current_service.organisation:
+    if (not current_service.organisation_type == Organisation.TYPE_NHS_LOCAL) or current_service.organisation:
         abort(403)
 
     form = AddNHSLocalOrganisationForm(organisation_choices=[
         (organisation.id, organisation.name)
         for organisation in Organisations()
-        if organisation.organisation_type == 'nhs_local'
+        if organisation.organisation_type == Organisation.TYPE_NHS_LOCAL
     ])
 
     search_form = SearchByNameForm()

app/models/organisation.py

@@ -9,15 +9,24 @@ from app.notify_client.organisations_api_client import organisations_client
 
 class Organisation(JSONModel):
 
+    TYPE_CENTRAL = 'central'
+    TYPE_LOCAL = 'local'
+    TYPE_NHS_CENTRAL = 'nhs_central'
+    TYPE_NHS_LOCAL = 'nhs_local'
+    TYPE_NHS_GP = 'nhs_gp'
+    TYPE_EMERGENCY_SERVICE = 'emergency_service'
+    TYPE_SCHOOL_OR_COLLEGE = 'school_or_college'
+    TYPE_OTHER = 'other'
+
     TYPES = (
-        ('central', 'Central government'),
+        (TYPE_CENTRAL, 'Central government'),
-        ('local', 'Local government'),
+        (TYPE_LOCAL, 'Local government'),
-        ('nhs_central', 'NHS – central government agency or public body'),
+        (TYPE_NHS_CENTRAL, 'NHS – central government agency or public body'),
-        ('nhs_local', 'NHS Trust or Clinical Commissioning Group'),
+        (TYPE_NHS_LOCAL, 'NHS Trust or Clinical Commissioning Group'),
-        ('nhs_gp', 'GP practice'),
+        (TYPE_NHS_GP, 'GP practice'),
-        ('emergency_service', 'Emergency service'),
+        (TYPE_EMERGENCY_SERVICE, 'Emergency service'),
-        ('school_or_college', 'School or college'),
+        (TYPE_SCHOOL_OR_COLLEGE, 'School or college'),
-        ('other', 'Other'),
+        (TYPE_OTHER, 'Other'),
     )
 
     ALLOWED_PROPERTIES = {

app/models/service.py

@@ -270,7 +270,7 @@ class Service(JSONModel):
 
     @property
     def shouldnt_use_govuk_as_sms_sender(self):
-        return self.organisation_type != 'central'
+        return self.organisation_type != Organisation.TYPE_CENTRAL
 
     @cached_property
     def sms_senders(self):
@@ -419,7 +419,7 @@ class Service(JSONModel):
 
     @property
     def needs_to_change_email_branding(self):
-        return self.email_branding_id is None and self.organisation_type != 'central'
+        return self.email_branding_id is None and self.organisation_type != Organisation.TYPE_CENTRAL
 
     @property
     def letter_branding_id(self):
@@ -599,7 +599,10 @@ class Service(JSONModel):
     def able_to_accept_agreement(self):
         return (
             self.organisation.agreement_signed is not None
-            or self.organisation_type in {'nhs_gp', 'nhs_local'}
+            or self.organisation_type in {
+                Organisation.TYPE_NHS_GP,
+                Organisation.TYPE_NHS_LOCAL,
+            }
         )
 
     @property