Stateless Hoff

[Riscky]

This is an idea we discussed IRL, but writing this down for posterity:

Currently Hoff relies on its state file being in sync with GitHub. #319 partially addresses this, but to completely keep the state in sync, we need to also be able to be able to pick up other missed webhooks.

The two things that we after the above linked PR only handle by webhook are:

• CommentAdded: Syncing the approval status could be done by scanning the comments and figuring out if the PR was approved. To make this simpler to scan, it would be nice if we always comment a thumbs-up on the merge comment. See Also use a reaction for confirmation when the PR ends up in the rebase queue #278;
• PullRequestEdited: We should also sync things like author and base branch;

Optionally, we could also hugely simplify (syncing) build statuses, see #255

With these changes in place, Hoff should be able to pick up where it left off, even if it missed a few webhooks, and even if it was down for a while.