xblock-mentoring: Add repository to requirements & installed apps -- For initial review, do not merge

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.