Redirect to original URL, if coming from *any* interesting page (i.e. NOT the homepage :) )

old behavior:
* always redirect to some page after login:
    * students;  account management page
    * teachers: coach reports page
    * admins: admin login page

current behavior:
* If you're NOT on the homepage when you click "login", after login return to that page
* If you're on the homepage, follow the old behavior logic

@wangguan59 can you try this out?