Tweak behavior of submit buttons on register and login pages

The buttons are not re-enabled if the registration/login submission succeeds
(and, therefore, the user is about to be directed to another page). Also, any
error message that is present does not disappear immediately before the page
redirects.
Merge pull request #406 from edx/gprice/register-button-enabling-tweak

Tweak behavior of submit buttons on register and login pages