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.

app/lib/ostatus/activity/deletion.rb

@@ -3,7 +3,9 @@
 class OStatus::Activity::Deletion < OStatus::Activity::Base
   def perform
     Rails.logger.debug "Deleting remote status #{id}"
-    status = Status.find_by(uri: id, account: @account)
+
+    status   = Status.find_by(uri: id, account: @account)
+    status ||= Status.find_by(uri: activitypub_uri, account: @account) if activitypub_uri?
 
     if status.nil?
       redis.setex("delete_upon_arrival:#{@account.id}:#{id}", 6 * 3_600, id)