Move branding info from model

This was an awkward property to have in a model class because
- it’s presentation not data about the object
- it munged together data from a different object, so it wasn’t well
  encapsulated

This commit moves the logic to the template, where it can reference the
`Organisation` and `User` models directly.

app/templates/support-tickets/branding-request.txt

@@ -1,4 +1,8 @@
-Organisation: {{ current_service.organisation.as_info_for_branding_request(current_user.email_domain) }}
+Organisation: {% if current_service.organisation -%}
+  {{ current_service.organisation.name }}
+{%- else -%}
+  Can’t tell (domain is {{ current_user.email_domain }})
+{%- endif %}
 Service: {{ current_service.name }}
 {{ url_for('main.service_dashboard', service_id=current_service.id, _external=True) }}