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_dashboard.py

@@ -106,14 +106,14 @@ def test_redirect_from_old_dashboard(
     mocker,
 ):
     mocker.patch('app.user_api_client.get_user', return_value=user)
-    expected_location = 'http://localhost/services/{}'.format(SERVICE_ONE_ID)
+    expected_location = '/services/{}'.format(SERVICE_ONE_ID)
 
     client_request.get_url(
         '/services/{}/dashboard'.format(SERVICE_ONE_ID),
         _expected_redirect=expected_location,
     )
 
-    assert expected_location == url_for('main.service_dashboard', service_id=SERVICE_ONE_ID, _external=True)
+    assert expected_location == url_for('main.service_dashboard', service_id=SERVICE_ONE_ID)
 
 
 def test_redirect_caseworkers_to_templates(
@@ -129,7 +129,6 @@ def test_redirect_caseworkers_to_templates(
         _expected_redirect=url_for(
             'main.choose_template',
             service_id=SERVICE_ONE_ID,
-            _external=True,
         )
     )