Bug 852536

Description of problem:
We intentionally do not count Backbone activity as session activity, so that sitting on a page using Backbone won't continually refresh your session and prevent the inactivity timeout from taking place.

However, this leaves a case where a user may be sitting on a page watching for a state change (perhaps an instance goes from "New" to "Running"), and, in that time, their session expires. The user currently receives no notice that this has happened, so they will continue watching for the state change, unaware that Backbone is no longer able to poll for updates.

What we think the best course of action here is to add a little code to detect when Backbone hits an expired session, and display a flash message on the page informing the user that Backbone will no longer automatically update page content.

We have to be careful here, because we do NOT want to display a general "Your session has expired!" message, or automatically redirect the user to the login page. That would lead to inconsistent behavior, where that message would only be displayed on pages that happen to use Backbone, which would likely bewilder users. So the message should be carefully worded and merely indicate that the page will no longer update automatically.