Maintainer mentorship

Open items:

1. This ignores reviewer-roulette. I figured it was best to get started and add that in future.
2. Buddy allocation. We have ten maintainers and eight trainees.
   1. If any maintainer doesn't want to be a mentor, let me know! Hopefully there are no more than two of you!
   2. Timezones. The existing maintainers are 3x Americas, 7x Europe. The trainees are 2x Americas, 3x Europe, and 2x Asia / Oceania. This should work out OK, but we probably want to consider Ash's and Thong's buddies first as their timezones are the trickiest.
3. Actually creating the issues, which we can do once this is merged, and we've figured out who's working with whom.

I've already created the trainee maintainer label.

Closes #3267 (closed).