Fix deletes not reaching every server that interacted with status (#15200)

Extract logic for determining ActivityPub inboxes to send deletes
to to its own class and explicitly include the person the status
replied to (even if not mentioned), people who favourited it, and
people who replied to it (though that one is still not recursive)

app/lib/status_reach_finder.rb

@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+class StatusReachFinder
+  def initialize(status)
+    @status = status
+  end
+
+  def inboxes
+    Account.where(id: reached_account_ids).inboxes
+  end
+
+  private
+
+  def reached_account_ids
+    [
+      replied_to_account_id,
+      reblog_of_account_id,
+      mentioned_account_ids,
+      reblogs_account_ids,
+      favourites_account_ids,
+      replies_account_ids,
+    ].tap do |arr|
+      arr.flatten!
+      arr.compact!
+      arr.uniq!
+    end
+  end
+
+  def replied_to_account_id
+    @status.in_reply_to_account_id
+  end
+
+  def reblog_of_account_id
+    @status.reblog.account_id if @status.reblog?
+  end
+
+  def mentioned_account_ids
+    @status.mentions.pluck(:account_id)
+  end
+
+  def reblogs_account_ids
+    @status.reblogs.pluck(:account_id)
+  end
+
+  def favourites_account_ids
+    @status.favourites.pluck(:account_id)
+  end
+
+  def replies_account_ids
+    @status.replies.pluck(:account_id)
+  end
+end