core.models.assignment_group: Added order_by_candidate_full_name() and

order_by_candidate_username() to the Manager and QuerySet. This is
both very useful shortcuts, and they mitigate the issues we will
encounter when we merge DevilryUserProfile into a custom Django user
model.

The tests is a proof of concept for #574.
docs.core_intro: Started on a core intro. See #573.