Remove attempts to access settings at import time

[David Evans]

There are currently two places (that I can find) in Django where settings are read at import time:

1. ​django/django/core/cache/__init__.py
2. ​django/django/contrib/staticfiles/urls.py

Case 1 is causing some people confusion when attempting to import things in wsgi.py, as the import breaks if it comes before DJANGO_SETTINGS_MODULE is defined. See for example: ​https://github.com/evansd/whitenoise/issues/31#issuecomment-104185649

This check on the contents of settings.CACHE seems to me to be superfluous as failing to define a default cache backend will trigger an InvalidCacheBackendError anyway when attempting to access it. I don't see that triggering ImproperlyConfigured on import adds much here.

Case 2 seems to be entirely redundant as it's only used for defining urlpatterns in that file and I can't find anything which uses this or any mention of it in the docs going back to v1.4. If I remove those lines the tests continue to pass, so I hoping this can be safely removed.

The coding style document ​suggests that such import-time evaluation of settings is to be avoided, and it's causing at least some real-world confusion, so it would be good to clean this up if possible.