Issue 5421: Autoenroll for already registered students

Ticket Link: http://atutor.ca/atutor/mantis/view.php?id=5421

Now, using the auto-enroll link, registered students as well as new users would be able to register for multiple courses specified in the link.

If the registered user is logged in, he would be shown only the list of courses and a confirm and cancel button.(The registration form wouldn't be shown)

I have also created a "If already registered, login here" link on registration.php which would allow users to login and then continue with the auto-enroll process.