Add organisation_type property to Service model

This will return the organisation_type of the service's organisation (if
there is one), or the organisation_type of the service if not.

app/models/service.py

@@ -32,7 +32,6 @@ class Service(JSONModel):
         'letter_contact_block',
         'message_limit',
         'name',
-        'organisation_type',
         'permissions',
         'prefix_sms',
         'research_mode',
@@ -409,6 +408,10 @@ class Service(JSONModel):
     def organisation_id(self):
         return self._dict['organisation']
 
+    @property
+    def organisation_type(self):
+        return self.organisation.organisation_type or self._dict['organisation_type']
+
     @cached_property
     def inbound_number(self):
         return inbound_number_client.get_inbound_sms_number_for_service(self.id)['data'].get('number', '')