Tell people to change their branding

In some cases it’s not appropriate for teams to have GOV.UK branding.
But they all start with it by default, if we can’t make a better guess.
We should be more explicit about this to reduce the number of teams
sending emails with the wrong branding.

app/models/service.py

@@ -417,6 +417,10 @@ class Service(JSONModel):
             return 'GOV.UK'
         return self.email_branding['name']
 
+    @property
+    def needs_to_change_email_branding(self):
+        return self.email_branding_id is None and self.organisation_type != 'central'
+
     @property
     def letter_branding_id(self):
         return self._dict['letter_branding']