Fix for issue 577: Display modal on failed ajax calls from expired sessions

This is in reference to https://github.com/MarkUsProject/Markus/pull/981 and https://github.com/MarkUsProject/Markus/issues/577

Rather than simply log the user out, a modal is displayed warning the user that the session has expired. From the modal, they can either close it to return to the page and look at what changes will be lost, or return to the login page. It has also been applied to all main pages. Refer to the screenshot attached.

![login-required](https://f.cloud.github.com/assets/817212/1069633/adf9cf28-1426-11e3-9d91-5d378a29be37.png)