xblock-mentoring: Add repository to requirements & installed apps

Master isn't ready yet to load the XBlock, but submitting this to start the review of the mentoring XBlock.

The XBlock can be found at https://github.com/gsehub/xblock-mentoring

@nedbat @cpennington Let me know if you see anything wrong, or if you'd like me to do anything differently.

`requirements/edx/github.txt` also says "If you open a pull request that adds a new dependency, you should notify: @jtauber to check licensing, one of @e0d, @jarv, or @feanil to check system requirements", so pinging you too. Let me know if you need anything.