Add federation relay support (#7998)

* Add federation relay support

* Add admin UI for managing relays

* Include actor on relay-related activities

* Fix i18n

db/migrate/20180711152640_create_relays.rb

@@ -0,0 +1,12 @@
+class CreateRelays < ActiveRecord::Migration[5.2]
+  def change
+    create_table :relays do |t|
+      t.string :inbox_url, default: '', null: false
+      t.boolean :enabled, default: false, null: false, index: true
+
+      t.string :follow_activity_id
+
+      t.timestamps
+    end
+  end
+end