Revamped + Enhanced Shibboleth support

* If a shib users type in their email on the regular login page,
  redirects them to /shib-login/
* Modify student.views.accounts_login to handle redirects
  generated by @login_required for courses that use shib for
  access control.
  Redirect those logins to /shib-login/?next=
Removing handling for ExternalAuthMap.MultipleObjectsReturned