Gitlab Omniauth provider

[bachp]

I would like to see the Omniauth Gitlab authentication strategy included by default.

The advantage is that it can be used with gitlab.com as well as on premise Gitlab installations.

[GUI]

omniauth-gitlab has been added to the v0.14.0 release. Documentation: https://api-umbrella.readthedocs.io/en/latest/server/admin-auth.html#gitlab