Add caching configuration

[keimlink]

The project should have a caching configuration. Preferably with a Redis backend for public sites, because it allows a more fine grained control of the cache. This means django-redis-cache and hiredis must be added to extras_require.

For development the django.core.cache.backends.dummy.DummyCache backend should be used by default.

The BACKEND, LOCATION, VERSION and OPTIONS settings of the default cache backend should be configurable using environment variables.

Also, the session backend should use the faster Redis cache:

SESSION_ENGINE = "django.contrib.sessions.backends.cached_db"