Make a model collection for organisations

This makes returning a user’s organisations consistent with how we
return their services.

app/models/organisation.py

@@ -1,7 +1,7 @@
 from flask import abort
 from werkzeug.utils import cached_property
 
-from app.models import JSONModel, ModelList
+from app.models import JSONModel, ModelList, SerialisedModelCollection
 from app.notify_client.email_branding_client import email_branding_client
 from app.notify_client.letter_branding_client import letter_branding_client
 from app.notify_client.organisations_api_client import organisations_client
@@ -204,6 +204,9 @@ class Organisation(JSONModel):
         return organisations_client.get_services_and_usage(self.id, financial_year)
 
 
-class AllOrganisations(ModelList):
-    client_method = organisations_client.get_organisations
+class Organisations(SerialisedModelCollection):
     model = Organisation
+
+
+class AllOrganisations(ModelList, Organisations):
+    client_method = organisations_client.get_organisations

app/models/user.py

@@ -11,7 +11,7 @@ from app.event_handlers import (
     create_set_user_permissions_event,
 )
 from app.models import JSONModel, ModelList
-from app.models.organisation import Organisation
+from app.models.organisation import Organisation, Organisations
 from app.models.webauthn_credential import WebAuthnCredentials
 from app.notify_client import InviteTokenError
 from app.notify_client.invite_api_client import invite_api_client
@@ -320,10 +320,7 @@ class User(JSONModel, UserMixin):
 
     @property
     def organisations(self):
-        return [
-            Organisation(organisation)
-            for organisation in self.orgs_and_services['organisations']
-        ]
+        return Organisations(self.orgs_and_services['organisations'])
 
     @property
     def organisation_ids(self):