Moves user activation away from just clicking on reset password

To following the link in the password reset email
Tests + Now subclass PasswordResetForm instead of copy

Changed to subclassing django's PasswordResetForm and
overriding clean_password() instead of copy/paste.
Less lines to worry about for diff-cover this way =)
Refactored tests
Merge pull request #84 from edx/feature/jbau/activation-after-password-reset-confirm

Moves user activation from just clicking on reset password to following the link in the password reset email