User registration prevents using password as username.

Many users erroneously set their password as their username with the original layout, because the username field directly followed the password field. Users may be accustomed to the common occurrence of a password confirmation field directly following the password field.
To fix the issue, I did the following:
   - Moved the existing username and real name form fields above the password field.
   - Added a password confirmation field directly following the password field.
   - Added two validations in the create_account handler in common/djangoapps/student/views.py, which confirm that the password and password confirmation fields match and that the password field does not match the username field.

Existing tests have also been edited to incorporate the added password confirmation field, and new tests added to check the added functionality.