Rewrite Curator

[f-f]

Here's a rewrite for the Curator tool, to hopefully make it more maintainable/stable.

Improvements going in with this PR:

• switch to a single shared bus for messages where everyone posts, instead of passing messages around between threads. This makes it much easier to hook new stuff.
• clone the relevant repos only when making changes to them instead of doing it at start. This is because then we can just trash the repo if there are issues instead of trying to cleanup (which still fails sometimes, and then we get into broken state)
• do only one PR a day to package-sets instead of one for every package. This is to reduce annoyance
• open PRs even for broken verifications, so we can inspect what's up. This means also keeping a list of "packages that are skipped", because then we can get green CI again
• the way we keep the above list is by reading the PR threads and looking for commands for the Curator. E.g. if the most recent release of simple-json doesn't work with the set then @spacchettibotti ban simple-json would make the Curator skip that version
• stop opening PRs if the state is broken, but post to the same one. This is also to reduce annoyance