If sidebar is closed, display flash messages over 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

Flash message service:
 * Add workaround for duplicate "invalid uname or pw" error message
 * When the sidebar is closed, pass the message to the host frame instead.

Fixes #886.