workshops/models.py: Add PermissionsMixin to the Person model
workshops/models.py: Add PermissionsMixin to the Person model

This mixin adds permissions handling to the model [1], which will make
it possible to control access with per-user permissions when we get to
that point.  It also adds is_superuser [2], which we can use out of
the gate to give admins full access.

[1]: https://docs.djangoproject.com/en/1.7/topics/auth/customizing/#custom-users-and-permissions
[2]: https://docs.djangoproject.com/en/1.7/topics/auth/customizing/#django.contrib.auth.models.PermissionsMixin.is_superuser

swcarpentry/amy,shapiromatron/amy,vahtras/amy,shapiromatron/amy,vahtras/amy,wking/swc-amy,pbanaszkiewicz/amy,pbanaszkiewicz/amy,swcarpentry/amy,pbanaszkiewicz/amy,wking/swc-amy,wking/swc-amy,swcarpentry/amy,shapiromatron/amy,wking/swc-amy,vahtras/amy