Display a message on the login page

In some cases, particularly when having to log the user out after
performing some action (e.g. password change), we want to display a
friendly message on the login screen to explain to the user why they
have been redirected to the login page.

This adds a function to do so, and uses it in a couple of places:
 - When updating one's own password using the Settings panel
 - Session time out
 - HTTP 401

Change-Id: Ie53c5552159304e1f1304ac6211b3accfd9aa623
Implements: blueprint messages-on-login-page
Merge "Display a message on the login page"