Detect Squarelet logout

[freedmand]

Should detect when you have signed out of squarelet periodically to sign out of documentcloud.

Here's MuckRock's implementation: https://github.com/MuckRock/muckrock/blob/master/muckrock/templates/accounts/check_session_iframe.html

[mitchelljkotler]

Before doing this I want to investigate a potentially better way to handle this - the MuckRock implementation has issues