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/test_errorhandlers.py

@@ -62,7 +62,7 @@ def test_csrf_redirects_to_sign_in_page_if_not_signed_in(client_request, mocker)
     client_request.logout()
     client_request.get_url(
         '/cookies',
-        _expected_redirect=url_for('main.sign_in', next='/cookies', _external=True),
+        _expected_redirect=url_for('main.sign_in', next='/cookies'),
     )