Removed useless model definitions.

Surprisingly, this commit doesn't change any behavior at all. When a
model is defined with the same name as another model in the same app,
the definition of the first class is bound to the name of the class
regardless of the definition of the second class.
Fixed test for models in non-installed apps.

Models are now attached to any application they're defined in. Since
not_installed was inside app_loading, these models were mistakenly
attached to app_loading. The test that used them passed accidentally
when run after EggLoadingTest because that class' tearDown() method
replaces apps.all_models['app_loading'] by a copy of itself, while
it should remain the same as apps.app_configs['app_loading'].models.
Fixed app registry cleanup in app-loading tests.
Renamed a couple models to prevent duplicate names.
Fixed #21711 -- Enforced unicity of model names.
Added an app-loading feature to the release notes.
Fixed #21675 -- Added app configs for contrib apps.
Added catalogs for apps that didn't have any translations.
Fixed #21718 -- Renamed has_app to is_installed.
Avoided catching ImproperlyConfigured on django.setup().

Fixed #21737. Thanks Florian for the report.

Also removed redundant imports in that file.
Changed parent class calls to super in Field subclasses
Fixed #21142: Dependency failures on unmigrated apps.
Fix wording of auth superuser post-migrate handler
Fixed #21745 - Feature check UniqueTest.test_unique_null
Merge pull request #2150 from manfre/ticket-21745

Fixed #21745 - Feature check UniqueTest.test_unique_null
Fixed #21746 - Fixed test_get_reverse_on_unsaved_object test.

Test will no longer attempt to insert multiple NULL rows in to a unique
field if the database doesn't support it.
Merge pull request #2151 from manfre/ticket-21746

Fixed #21746 - Fixed test_get_reverse_on_unsaved_object test.
Make test stronger to hopefully fix odd Jenkins failure