Add eggs template loader to TEMPLATE_LOADERS

Dependencies installed via setup.py stay in their eggs and their
templates won't be found by Django. This caused a
TemplateNotFoundError when users were trying to use
Django's admin to set categories.
Using the admin is unsupported, but this still prevents a similar error
from happening when we use other dependencies that ship templates. It
also shouldn't slow down production use.

Fixes #761