Show login again instead of JSON if CSRF check fails

Previously a JSON error page was shown to the user in-case the CSRF token was not valid. This was confusing and prevented people from login.

With this at least the login page is shown again and not a JSON error message. I consider this as sufficient since adding a new error page just for this sake would uneededly make lib/base.php even more cluttered and this is a edge-case which optimally should anyways not happen that often.

This can be tested by opening the login page, then clearing the cookies, and trying to login.

@PVince81 As discussed.
@karlitschek Might be worth backporting to stable7.
I wanted to preserve the username in the login form at least, so the user has to type less.