feat: allow anyone to add basic labels, via comments

[kim-em]

Not everyone contributes to Mathlib by first asking for write permission: see #7339 for an example.

For those users, it's nice if it is at least possible to change awaiting-review and awaiting-author labels.

This PR copies across the existing functionality for this from Std.

---

[mathlib-bors[bot]]

:v: semorrison can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.

[grunweg]

Coming here for PR triage: is this PR still relevant? (We certainly have to functionality now.)