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_templates.py

@@ -580,7 +580,6 @@ def test_caseworker_redirected_to_set_sender_for_one_off(
             'main.set_sender',
             service_id=SERVICE_ONE_ID,
             template_id=fake_uuid,
-            _external=True,
         ),
     )
 
@@ -633,7 +632,6 @@ def test_user_with_only_send_and_view_redirected_to_set_sender_for_one_off(
             'main.set_sender',
             service_id=SERVICE_ONE_ID,
             template_id=fake_uuid,
-            _external=True,
         ),
     )
 
@@ -1207,7 +1205,6 @@ def test_choosing_to_copy_redirects(
         _expected_redirect=url_for(
             'main.choose_template_to_copy',
             service_id=SERVICE_ONE_ID,
-            _external=True,
         ),
     )
 
@@ -1722,7 +1719,6 @@ def test_should_redirect_when_saving_a_template(
             '.view_template',
             service_id=SERVICE_ONE_ID,
             template_id=fake_uuid,
-            _external=True,
         ),
     )
     mock_update_service_template.assert_called_with(
@@ -1753,7 +1749,6 @@ def test_should_edit_content_when_process_type_is_priority_not_platform_admin(
             '.view_template',
             service_id=SERVICE_ONE_ID,
             template_id=fake_uuid,
-            _external=True,
         )
     )
     mock_update_service_template.assert_called_with(
@@ -1956,7 +1951,6 @@ def test_removing_placeholders_is_not_a_breaking_change(
             'main.view_template',
             service_id=SERVICE_ONE_ID,
             template_id=fake_uuid,
-            _external=True,
         ),
     )
     assert mock_update_service_template.called is True
@@ -2066,7 +2060,6 @@ def test_should_redirect_when_saving_a_template_email(
             '.view_template',
             service_id=SERVICE_ONE_ID,
             template_id=fake_uuid,
-            _external=True,
         ),
     )
     mock_update_service_template.assert_called_with(
@@ -2183,7 +2176,6 @@ def test_should_redirect_when_deleting_a_template(
             '.choose_template',
             service_id=SERVICE_ONE_ID,
             template_folder_id=parent,
-            _external=True,
         )
     )