Record deleted(by mod) status to prevent re-appear (#10732)

* Record deleted(by mod) status to prevent re-appear

* Move to Tombstone

* Add missing migration script

app/models/tombstone.rb

@@ -4,11 +4,12 @@
 #
 # 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
+#  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
+#  by_moderator :boolean
 #
 
 class Tombstone < ApplicationRecord