Support undo activities, dont remove accepted inbox items

[RangerMauve]

• fixed a bug where rejected follows were actually accepted
• accepted activities no longer get removed
• added code path for Undo activities
• checks that the same author that made the activity made the undo activity
• removes the activity from the inbox
• if it was a follow request, removes it.

[catdevnull]

not familiar with the ActivityPub stuff but otherwise looks good

[akhileshthite]

Looks good to me!
Do we need to have tests for this in apsytem.test.ts; performUndo removes activity and follower for a Follow activity?

[fauno]

my expectation was for the undo/delete activities to remove the activity they reference, are always honored, so they aren't stored on the inbox for approval/rejection (ie. i don't approve you removing a reply).

but webhooks need to be notified in case i already did something with the activity. in the moderation queue we're storing activities with different states, so if someone undoes/deletes an activity, the moderation queue needs to be notified to change its state.

this seems to be the case because the onapproved hook is dispatched, but auto-approval is required to work properly.

[fauno]

Thanks!