CRAFTERCMS-392: Redirect to sign-in page when session times out

If a user is editing a form and the session ticket expires, the user will be prompted to choose between:
1) Leaving the page
2) Staying on the page

The user should stay on the page so he doesn't lose work and sign back into the application through the main window.

If the user chooses to leave the page, he may get a browser sign-in prompt (FF and IE) before being redirected to the sign-in page. Once the user has been redirected to the sign-in page, he is expected to sign in there and then and not go back to the main window (if the user does go back to the main window, he will not be able to work with the form anymore since the child window stores references to objects in the parent window).