Add ability to use email in Gitlab authentication

Older versions of Gitlab API v3 only allow authentication using email instead
of "login" parameter that Review board currently uses.

See https://github.com/gitlabhq/gitlabhq/commit/559e83d30004e0c41a30f4ce3463f695eb7e26a1

Testing Done:
Unit tests pass. Tested authentication with email againts different versions of
Gitlab.

Reviewed at https://reviews.reviewboard.org/r/5591/
Merge branch 'release-1.7.x' into release-2.0.x