To prevent an OAuth2 covert redirect vulnerability, this commit adds and
uses an alias for the GitHub and BitBucket OAuth2 callback URLs to the
following paths:

GitHub: /users/auth/-/import/github
Bitbucket: /users/auth/-/import/bitbucket

This allows admins to put a more restrictive callback URL in the OAuth2
configuration settings. Instead of https://example.com, admins can now use:

https://example.com/users/auth

It's possible but not trivial to change Devise and OmniAuth to use a
different prefix for callback URLs instead of /users/auth. For now,
aliasing the import URLs under the /users/auth namespace should suffice.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56663