Validate GitHub collaborator upon Accepting a specific ensemble, or have as pre-requisite to accept

[tedyoung]

Follow on to #73.

Need a way to ensure someone is properly set up with the repository they'll be using. Since there are multiple Ensembles, each one has its own repository, and so someone may be set up to work for one Ensemble, but not the other.

We can either add the person as a collaborator upon "Accept", or only show eligible ensembles.

I'd prefer the latter, to prevent someone from joining an ensemble when they're not prepared.

Bonus: ensure that they're not only a collaborator for the repo, but have used mob.sh to correctly do a "mob start" and "mob next" to a branch of that repo. Have them do a mob start --branch <their username> and then the usual start, edit participants.txt, mob next. Then verify there's a commit on that branch, and then delete that branch.