Stop using _external=True in tests

It looks like, by default, Flask no longer makes full URLs, for example
`https://example.com/path`. Instead it does `/path`. This will still
work fine, and if anything is better because it reduces the number of
bytes of HTML we are sending.

It won’t mean that requests go over `http` instead of `https` without
the protocol because we set the appropriate HSTS header here:
0c57da7781/ansible/roles/paas-proxy/templates/admin.conf.j2 (L11)

This commit changes all our tests to reflect that URLs no longer have
the protocol and domain in them. `_external=True` is Flask’s way of
saying whether a URL should be generated with the domain and protocol
(`True`) or without it (`False`).

Again, I can’t find the changelog or diff where this was introuduced,
but if you’d like to go spelunking then here’s a starting point:
50374e3cfe/src/flask/helpers.py (L192)

tests/app/main/views/test_add_service.py

@@ -148,7 +148,6 @@ def test_should_add_service_and_redirect_to_tour_when_no_services(
             'main.begin_tour',
             service_id=101,
             template_id="Example%20text%20message%20template",
-            _external=True,
         ),
     )
     assert mock_get_services_with_no_services.called
@@ -270,7 +269,6 @@ def test_should_add_service_and_redirect_to_dashboard_when_existing_service(
         _expected_redirect=url_for(
             'main.service_dashboard',
             service_id=101,
-            _external=True,
         )
     )
     assert mock_get_services.called