Fix relative URLs in support tickets

When we get a support ticket we put a link to the service at the end.

As part of 8b7f2fbf04 we accidentally made
these URLs relative, rather than absolute. This means they aren’t made
into links by email clients or Zendesk.

This commit fixes the links to include the domain again.

app/templates/support-tickets/support-ticket.txt

@@ -1,5 +1,5 @@
 {{ content }}
 {% if current_service -%}
 Service: "{{ current_service.name }}"
-{{ url_for('main.service_dashboard', service_id=current_service.id) }}
+{{ url_for('main.service_dashboard', service_id=current_service.id, _external=True) }}
 {% endif %}

tests/app/main/views/test_feedback.py

@@ -237,6 +237,7 @@ def test_passes_user_details_through_flow(
         url_for(
             'main.service_dashboard',
             service_id=SERVICE_ONE_ID,
+            _external=True,
         ),
         ''
     ])