If sidebar is closed, display flash messages in the host frame

In sidebar:
 * Save our H instance as window.annotator
 * Add isOpen() method to test if the sidebar is open
 * Removed unused draft dependency from controller

In host document:
 * Add support for showing the flash messages
 * Save the Annotator class as @Annotator
 * Redefine the notification level constants in Annotator.Notification
   so that they are consistent with the ones used in the sidebar

Flash message service:
 * Add workaround for duplicate "invalid uname or pw" error message,
   and also swallow the "your submission was invalid" in this case.
 * When the sidebar is closed, pass the message to the host frame
   instead.

Fixes #886.

This PR obsoletes #887.