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/uploads/test_upload_contact_list.py

@@ -432,7 +432,6 @@ def test_save_contact_list(
         _expected_redirect=url_for(
             'main.uploads',
             service_id=SERVICE_ONE_ID,
-            _external=True,
         )
     )
     mock_get_metadata.assert_called_once_with(
@@ -721,7 +720,6 @@ def test_delete_contact_list(
         _expected_redirect=url_for(
             'main.uploads',
             service_id=SERVICE_ONE_ID,
-            _external=True,
         )
     )
     mock_delete.assert_called_once_with(

tests/app/main/views/uploads/test_upload_letter.py

@@ -481,7 +481,6 @@ def test_uploaded_letter_preview_redirects_if_file_not_in_s3(
             'main.view_notification',
             service_id=SERVICE_ONE_ID,
             notification_id=fake_uuid,
-            _external=True
         )
     )
 
@@ -636,7 +635,6 @@ def test_send_uploaded_letter_sends_letter_and_redirects_to_notification_page(
             'main.view_notification',
             service_id=SERVICE_ONE_ID,
             notification_id=fake_uuid,
-            _external=True
         )
     )
     mock_send.assert_called_once_with(
@@ -670,7 +668,6 @@ def test_send_uploaded_letter_redirects_if_file_not_in_s3(
             'main.view_notification',
             service_id=SERVICE_ONE_ID,
             notification_id=fake_uuid,
-            _external=True
         )
     )