Block login/logout loop

[adehnert]

Since fixing #68, it has been possible to get in a ~loop with the login button: if you click the "login" link in the upper right of ​https://asa.mit.edu/accounts/logout/, you'll get logged in, redirected to your previous page (the logout page), and logged back out. This is stupid. One option would be to refuse redirect to the logout page from the login page. Another would be to override the login link on the logout page so it doesn't pass the "?next=" parameter.

See also #35 for vaguely similar logout button issues.