Pleroma federation fixes and hidden RSVP functionality

[lowercasename]

Bugfixes

This PR fixes the longstanding issues with Pleroma federation, which all stem from the fact that Pleroma generally has a stricter interpretation of the ActivityPub spec, and in other ways just has its own ideas about things like Question objects, but Gathio's AP implementation wasn't originally developed against a Pleroma instance. This fixes:

• The bug where RSVP Polls couldn't be replied to from Pleroma (because Pleroma expects to see the number of current votes for a poll in the poll object)
• A bug where Gathio wouldn't receive and repost messages mentioning the event Actor when sent from a Pleroma instance
• One click unattending also works.

There may still be bugs with:

• Unboosting Pleroma boosts when a comment is deleted on Gathio
• Replying to Pleroma DMs with a message explaining that the user should mention the event actor instead

These will be handled in a separate PR, if necessary.

Features

This also starts the AP migration to Typescript, and adds a new feature - optionally hidden RSVPs! Currently this only accessible via ActivityPub interaction, where in the RSVP poll you can now choose one of three options:

If you choose to be hidden from the public list, the public list will look like this for regular users:

...and like this for the event editors:

To do

• Allow hidden RSVP from Gathio UI

[dariusk]

Incredible work here!