Refresh OAuth tokens

Closes #1037 (closed)

This removes a lot of unused arguments on GitlabProvider and GitlabOAuthProvider, as they were quite confusing and probably introduced bugs when using GitLab instances other than gitlab.com. This fixes some unit tests that were not testing anything properly or could be influenced by various settings, removes some unused mocks, and replaces some mocks with responses. The providers now only require HTTP requests when they actually have some data to get from the request, not when they just want to generate URLs, so that manage.py commands will be able to refresh OAuth tokens.