Upgrade to Django 1.7

Upgrading to Django 1.7 requires the following:

- [x] [Upgrading Django to a newer version](https://docs.djangoproject.com/en/1.7/howto/upgrade-version/).
- [x] [Upgrading from South](https://docs.djangoproject.com/en/1.7/topics/migrations/#upgrading-from-south).
- [x] [Upgrading to django-registration-redux](http://stackoverflow.com/a/25836530/2422398), since django-registration is no longer maintained and doesn't work with 1.7.
- [x] Upgrading to dj-static 0.0.6, which added Django 1.7 support in https://github.com/kennethreitz/dj-static/commit/0b3e3a4b565d501e104ee55734bf9d60f81e3ec6.
- [x] Upgrading to djrill 1.2, which added Django 1.7 support.

While crispy-forms 1.5.0 has been released, its [changelog](https://github.com/maraujop/django-crispy-forms/blob/dev/CHANGELOG.md#150) does not seem to indicate that it contains any Django 1.7 or high-priority security fixes, so we'll stay at 1.4 for now. Note that running the tests with warnings turned on does raise a `RemovedInDjango19Warning` from crispy-forms telling us to use `django.utils.lru_cache` instead of `memoize`, but we've got plenty of time to upgrade and I fear that upgrading now may break our `directory/table_inline_formset.html` template, which I don't want to deal with.

django-csp is still at 2.0.3, so nothing to upgrade there.

bleach is still at 1.4, so nothing to upgrade there.