feat: Notify when a re-noted note is edited

[dakkar]

Summary

1. Notify a user when a note they re-noted was edited.
2. When editing a note, send the "update" activity also to all instances that re-noted it.

Longer version

Scenario:

• @a@one.example.com creates a note
• @b@two.example.com re-notes it (or quote-re-notes it)
• @a@one.example.com edits the note and the update activity reaches two.example.com

It would be very useful if @b@two.example.com got notified of the edit, because the new contents may not be something they want to have re-noted

(I think Sharkey doesn't do this currently, feel free to just close this issue otherwise!)

I understand that, in general, the originating instance (one.example.com in the scenario) may not send the edit/update to all instances that have re-noted the original note; it would be nice if Sharkey did, though: we already track all re-notes, so we know who to send the edit/update to.