Add tombstones for remote statuses (#9830)

* Add Tombstone model to remember object deletion

* Do not recreate a status if it has been deleted

* Record Tombstone for remote deleted items

Also, only record deleted items from same-host actors

* Clear an user's tombstones when their key change

app/models/tombstone.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: tombstones
+#
+#  id         :bigint(8)        not null, primary key
+#  account_id :bigint(8)
+#  uri        :string           not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+
+class Tombstone < ApplicationRecord
+end