Serialize ActivityPub alternate link into OStatus deletes, handle it (#4730)

Serialize ActivityPub alternate link into OStatus deletes, handle it (#4730)

Requires moving Atom rendering from DistributionWorker (where
`stream_entry.status` is already nil) to inline (where
`stream_entry.status.destroyed?` is true) and distributing that.

Unfortunately, such XML renderings can no longer be easily chained
together into one payload of n items.