new port: security/rubygem-omniauth-gitlab

Ruby gem that implements the OmniAuth strategy of GitLab
for usage in rubygem-onmiauth.

WWW: https://github.com/linchus/omniauth-gitlab

PR: 199779
Differential Revision: https://reviews.freebsd.org/D2502
Submitted by: Torsten Zuehlsdorff <ports@toco-domains.de>
Approved by: mat (mentor)