Remove stale cookies

[fisx]

on login and logout, all aula cookies should be marked as stale and removed from the browser.

until i removed my cookies manually, i had the problem that the first two of the four links in the pop-menu were running into broken pages. i haven't taken the time to investigate because i think it's a good idea in general to not use stale cookies.

(somewhat related, but not too closely: #219.)