Cloud: Create GitLab OAuth signin

Created by: unknwon

Use [email protected] to own the GitLab OAuth application for Sourcegraph since GitLab does not support register OAuth application for a group/organization.

Context in slack

TODO

• Create OAuth application on GitLab.com
• Enable sign in with GitLab OAuth

Blocking on https://github.com/sourcegraph/sourcegraph/issues/15332 to be resolved.