Django 1.6 support

  - Test on Django 1.6

  - New password reset uses base 64
    
    Django ticket #14881 switched to base64 encoding user ids instead of the old base36. This patch makes our view work with either format.

  - UNUSABLE_PASSWORD was removed in Django 1.6
    
    Now we have to use an is_password_usable function that works with both Django versions. We can't use django.contrib.auth.hashers.is_password_usable either, because it not only checks against the unusable password, but checks for a valid hasher too. We need different error messages in those cases.
    
    See Django ticket #20079.