Avoid getting stuck in a loop in CAS login
498ea53 made it so that the #/login URL fragment was prioritised over the token
params in the query string; unfortunately that also means that CAS login gets
stuck in a loop where you always get redirected back to the login view.
Prioritising the URL fragment over the token params may or may not be the
correct thing to, but I also think it's incorrect that we ask the CAS server to
redirect us back to #/login. Accordingly, the easiest fix here is just to clear
the URL fragment before redirecting to CAS.
