Add platform admin setting for letter branding

Does two main things:
- defines what ‘brands’ we support, in terms of the ID that DVLA use
- adds a form to choose which branding a service uses (currently
  platform admin only, like email branding)

By doing this we will be able to (with some more work) preview and send
letters with a variety of different branding.

Story: https://www.pivotaltracker.com/story/show/143506905

app/main/forms.py

@@ -538,6 +538,20 @@ class ServiceBrandingOrg(Form):
     )
 
 
+class LetterBranding(Form):
+
+    def __init__(self, choices=[], *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.dvla_org_id.choices = choices
+
+    dvla_org_id = RadioField(
+        'Which logo should this service’s letter have?',
+        validators=[
+            DataRequired()
+        ]
+    )
+
+
 class Whitelist(Form):
 
     def populate(self, email_addresses, phone_numbers):

app/main/views/service_settings.py

@@ -26,7 +26,8 @@ from app.main.forms import (
     ServiceReplyToEmailFrom,
     ServiceSmsSender,
     ServiceLetterContactBlock,
-    ServiceBrandingOrg
+    ServiceBrandingOrg,
+    LetterBranding,
 )
 from app import user_api_client, current_service, organisations_client
 
@@ -41,7 +42,10 @@ def service_settings(service_id):
         organisation = None
     return render_template(
         'views/service-settings.html',
-        organisation=organisation
+        organisation=organisation,
+        letter_branding=dict(
+            current_app.config['LETTER_BRANDING']
+        ).get(current_service.get('dvla_org_id', '001'))
     )
 
 
@@ -314,6 +318,28 @@ def service_set_branding_and_org(service_id):
     )
 
 
+@main.route("/services/<service_id>/service-settings/set-letter-branding", methods=['GET', 'POST'])
+@login_required
+@user_has_permissions(admin_override=True)
+def set_letter_branding(service_id):
+
+    form = LetterBranding(choices=current_app.config['LETTER_BRANDING'])
+
+    if form.validate_on_submit():
+        service_api_client.update_service(
+            service_id,
+            dvla_organisation=form.dvla_org_id.data
+        )
+        return redirect(url_for('.service_settings', service_id=service_id))
+
+    form.dvla_org_id.data = current_service.get('dvla_organisation', '001')
+
+    return render_template(
+        'views/service-settings/set-letter-branding.html',
+        form=form,
+    )
+
+
 def get_branding_as_value_and_label(organisations):
     return [
         (organisation['id'], organisation['name'])