Fix edX name hardcoded in password reset email

Updating commits from #5863 with a bunch of needed fixes, added tests, and squashed commits.

[OSPR-187](https://openedx.atlassian.net/browse/OSPR-187)
This import is here because it's here in the Django 1.4.5 source code: https://github.com/django/django/blob/67a937c2c24b7eee8be13a1d3faff595d8f45839/django/contrib/auth/forms.py#L221

Moving the import to the top of the file causes a bunch of errors. So I'm leaving this here, since our goal is to simply slightly modify the existing behavior of the PasswordResetForm.save() function.