Fix Bitbucket importer on GitLab.com

Currently our Bitbucket importer is not working, possibly because the redirect_uri is not configured properly: https://gitlab.com/gitlab-com/support-forum/issues/1373#note_20343836. @marin has the credentials.

I think the error is one of two things:

1. Our OAuth2 credentials redirect_uri is not set to the right URL: https://gitlab.com/import/bitbucket/callback
2. The url field inside our Bitbucket credentials in gitlab.rb mucks things up with the new BitBucket API 2.0 importer

I think it is the second point, but we should double check the first point.