Allow rooms to have an optional access code (#646)

db/migrate/20190711192033_add_password_digest_to_rooms.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddPasswordDigestToRooms < ActiveRecord::Migration[5.0]
+  def change
+    add_column :rooms, :access_code, :string, null: true, default: nil
+  end
+end