Provide a nicer selector for cache backends compatible with settings.CACHES.

The cache backend selector in General Settings is now much nicer,
providing a dropdown of possible cache types and an appropriate field
for entering the hosts or cache location.

Two entries are provided by default: Memcached and File cache.

If running with DEBUG = True, then "Local memory cache" will also be
available, making it easier to test and debug things.

If some other cache backend is used, then "Custom" will be an option as
well, and will leave the cache settings untouched.

This moves us away fully from the old Django CACHE_BACKEND setting.

Tested each cache backend and verified that the correct cache settings
were stored in siteconfig and were being loaded out correctly.