Method to flush all user accounts created >24hrs that have not been validated

[jlumbroso]

This would make it easier to clean up after a spam attack. @andyguna

[lgoodridge]

Added in version 1.2.5. You can either use uniauth.utils.flush_old_tmp_users, or the flush_tmp_users django command (e.g. python manage.py flush_tmp_users).

Both accept an optional days argument specifying how old temporary users have to be get deleted, but the default for both is 1.