Make AddHideNotificationsToMute Concurrent

It's not clear how much this will benefit instances in practice, as the
number of mutes tends to be pretty small, but this should prevent any
blocking migrations nonetheless.

db/migrate/20170716191202_add_hide_notifications_to_mute.rb

@@ -1,5 +1,13 @@
 class AddHideNotificationsToMute < ActiveRecord::Migration[5.1]
-  def change
-    add_column :mutes, :hide_notifications, :boolean, default: true, null: false
+  include Mastodon::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  def up
+    add_column_with_default :mutes, :hide_notifications, :boolean, default: true, allow_null: false
+  end
+  
+  def down
+    remove_column :mutes, :hide_notifications
   end
 end