JH/migrate omniauth cas3 into JH

What does this MR do and why?

Related issue: #2156

Upstream MR: Removed cas3 gem and related code

In order to continue keeping cas3 support for some Chinese users, we migrate the omniauth-cas3 from upstream into JH release.

This MR did these things for the migration:

1. Move omniauth-cas3 gem from gitlab/vendor into gitlab/jh/vendor, update all Gemfile
2. Migrate cas3 related files from upstream into JH proper place, which depends on the prepend_mod
3. Migrate cas3 related tests files from upstream into JH proper place
4. Move some cas3 doc files from upstream into JH proper place
5. Move cas3 some ci configs from upstream into JH proper place

Screenshots

Guide Doc and screenshots can be found here: How to setup cas3 development env

/cc @chaomao