Add the ability for Rooms to have a URL.

The URL is shown on the schedule and the event/proposal page.
The URL is only when a user is logged in and registered.

db/migrate/20200710215300_add_url_to_rooms.rb

@@ -0,0 +1,5 @@
+class AddUrlToRooms < ActiveRecord::Migration[5.2]
+  def change
+    add_column :rooms, :url, :text
+  end
+end