Fixed #21651 -- Remove recapitalize.

It was not used inside Django, is not tested or documented. Consequently
remove without deprecation path.

Thanks to @vajrasky for bringing it to our attention.
Used a more translator-friendly format in admin sites.py
Refactored INSTALLED_APPS overrides.

* 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.
Added utilities to alter settings containing lists of values.
Dropped AppCache._empty test utility.

It's now easier to achieve the same effect with override_settings.
Dropped AppCache._without_app test utility.

It's now easier to achieve the same effect with override_list_settings.
Dropped AppCache._with_app test utility.

It's now easier to achieve the same effect with override_list_settings.

I'm finally removing test_missing_app as it is basically impossible to
reach this situation with public methods of the new app cache.