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

db/migrate/20190117114553_create_tombstones.rb

@@ -0,0 +1,12 @@
+class CreateTombstones < ActiveRecord::Migration[5.2]
+  def change
+    create_table :tombstones do |t|
+      t.belongs_to :account, foreign_key: { on_delete: :cascade }
+      t.string :uri, null: false
+
+      t.timestamps
+    end
+
+    add_index :tombstones, :uri
+  end
+end