reply cache to delete our DM if they delete

[fasiha]

Previously, if we received a notification that someone deleted a toot, we asked our server for our most recent sent toots to see if we could find a reply to the deleted toot. If we found such a reply, we'd delete it (so it's not cluttering the other user's DMs).

Most of the time this does nothing because

1. the vast majority of deletes are of messages that we haven't responded to (i.e., either with no images or with captions).
2. But also because we can only ask the server for our most recent messages and we might have sent a reply to the now-deleted a while ago.

Instead of burdening the server to do this unlikely-to-be-useful search, this PR proposes an alternative: we store in memory a cache mapping other people's message IDs to our replies. When we DM someone about missing captions, it creates an entry in the cache. If we get a delete, we check the cache for the deleted message ID and if we find a hit, we delete our message and delete the entry from the cache.

We don't want the cache to grow infinitely so we cap it at some reasonable value (10'000 entries in this PR), and every 12 hours we run a cleanup operation to bring it to that size if it's grown beyond it.

Upsides:

• we don't burden the server
• we will likely detect and delete even months-old replies
  • (assuming the server stays running that long)

Downsides:

• the cache is in-memory and not persisted to disk. We lose the cache every time the server reboots (which is every few days recently, since I'm adding features).
  • we could save the cache to disk every few hours and on server shutdown but I don't want to have a random list of people's message IDs (that botsin.space has assigned) on the Glitch server, seems like a privacy 🤢

Bonus:

• we now also listen to edit messages and delete our DM if someone edits their toot and adds captions!

Potential enhancements:

• later once we detect missing captions in people's boosts, we should detect if someone has unboosted and delete our DM to them too
• TODO now: delete from our cache when someone favorites our DM

(My overall goal to reduce server load is so we can turn on boost analysis 😇)