Eliminated the app_ prefix that was more confusing than useful.

Previously the _apps/models_loaded flags could remain set to False if
set_installed_apps exited with an exception, which is going to happen as
soon as we add tests for invalid values of INSTALLED_APPS.

Thanks thepapermen for the report and Carl Meyer for the review.

Fixed a crash when executing changepassword command when the user object
representation contained non-ASCII characters.

Removed redundant line

Thanks David Larlet for the suggestion.

Also fixed some Sphinx warnings and improved ReST markup.

Used @cached_property for request.GET and COOKIES

- use code-block:: bash
- prefix the command with $

refs #19321

Thanks carrie.hazelton at altamiracorp.com for the report.

Thanks alex_koval for the report.

Thanks Tim for the review.

Typo fix for exception message.

GEOS 3.3 added crosses, disjoint, overlaps, touches and within
predicates on prepared geometries.

Thanks Robert Scott for the report.

Also renamed app_cache to apps and "app cache" to "app registry".

Deprecated AppCache.app_cache_ready() in favor of Apps.ready().

Invalidate properly the cache whenever all_models or app_configs change.
This fixes some isolation issues in the test suite.

Previously, this would corrupt the settings, because __exit__ isn't
called when __enter__raises an exception.

It's now easier to achieve the same effect with modify_settings or
override_settings.

* Introduced [un]set_installed_apps to handle changes to the
  INSTALLED_APPS setting.
* Refactored [un]set_available_apps to share its implementation
  with [un]set_installed_apps.
* Implemented a receiver to clear some app-related caches.
* Removed test_missing_app as it is basically impossible to reproduce
  this situation with public methods of the new app cache.