Setup salesforce Omniauth with GitLab.com

With https://gitlab.com/gitlab-org/gitlab-ce/issues/57077#note_169752576, we are being asked to setup Omniauth for GitLab.com with Salesforce. There is a promotion where we are looking to go live by May 29 so we have a clear understanding of expected dates. For infra, we should look to have an initial setup running next week (week of May 20).

Documentation: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27834/diffs?commit_id=97fa2bf90da6489e84b143f4d7b00395b634ffc7

Todo

• Disable Salesforce oauth sign-in source on staging, for consistency with other providers