Move branding choices logic into utility module

This was a lot of code to be in a form and it's going to get even
more complicated with email branding pools. Moving it out means we
can also simplify the tests that target this code.

app/utils/branding.py

@@ -0,0 +1,53 @@
+from app.models.organisation import Organisation
+
+
+def get_available_choices(service, branding_type):
+    if branding_type == "email":
+        organisation_branding_id = service.organisation.email_branding_id if service.organisation else None
+        service_branding_id = service.email_branding_id
+        service_branding_name = service.email_branding_name
+    elif branding_type == "letter":
+        organisation_branding_id = service.organisation.letter_branding_id if service.organisation else None
+        service_branding_id = service.letter_branding_id
+        service_branding_name = service.letter_branding_name
+
+    if (
+        service.organisation_type == Organisation.TYPE_CENTRAL
+        and organisation_branding_id is None
+        and service_branding_id is not None
+        and branding_type == "email"
+    ):
+        yield ('govuk', 'GOV.UK')
+
+    if (
+        service.organisation_type == Organisation.TYPE_CENTRAL
+        and service.organisation
+        and organisation_branding_id is None
+        and service_branding_name.lower() != 'GOV.UK and {}'.format(service.organisation.name).lower()
+        and branding_type == "email"
+    ):
+        yield ('govuk_and_org', 'GOV.UK and {}'.format(service.organisation.name))
+
+    if (
+        service.organisation_type in {
+            Organisation.TYPE_NHS_CENTRAL,
+            Organisation.TYPE_NHS_LOCAL,
+            Organisation.TYPE_NHS_GP,
+        }
+        and service_branding_name != 'NHS'
+    ):
+        yield ('nhs', 'NHS')
+
+    if (
+        service.organisation
+        and service.organisation_type not in {
+            Organisation.TYPE_NHS_LOCAL,
+            Organisation.TYPE_NHS_CENTRAL,
+            Organisation.TYPE_NHS_GP,
+        }
+        and (
+            service_branding_id is None
+            or service_branding_id != organisation_branding_id
+        )
+    ):
+        yield ('organisation', service.organisation.name)