Reverted 4a56a93c.

Currently such overrides aren't reflected in the app cache.

It would  be possible to handle them. But that doesn't look like a very
good API. It makes it complicated to express "add this app" and "remove
this app", which are the most common operations on INSTALLED_APPS.

- Tested consistency the current app_configs instead of INSTALLED_APPS.
- Considered applications added with _with_app as available.
- Added docstrings.

Except the app cache code and a few specific tests, of course.

Removed some exception masking in the comments app that was harmful and
couldn't be preserved easily.

This avoids possible confusion with register_model.

Thanks Marc Tamlyn for the suggestion.

Used the app cache's get_app_configs() method instead.

Took this opportunity to change get_app[s] to only consider applications
containing a model module as that seems slightly more backwards compatible.

Since callers that care about models pass only_with_models_module=True,
this has very few consequences. Only AppCommand needed a change.

Adjusted several tests that used it to add apps to the app cache and
then attempted to remove them by manipulating attributes directly.

Also renamed invalid_models to invalid_models_tests to avoid clashing
application labels between the outer and the inner invalid_models
applications.

It was called _populate() before I renamed it to populate(). Since it
has been superseded by populate_models() there's no reason to keep it.

Removed the can_postpone argument of load_app() as it was only used by
populate(). It's a private API and there's no replacement. Simplified
load_app() accordingly. Then new version behaves exactly like the old
one even though it's much shorter.

First stage imports app modules. It doesn't catch import errors. This
matches the previous behavior and keeps the code simple.

Second stage import models modules. It catches import errors and retries
them after walking through the entire list once. This matches the
previous behavior and seems useful.

populate_models() is intended to be equivalent to populate(). It isn't
wired yet. That is coming in the next commit.

Since applications that aren't installed no longer have an application
configuration, it is now always True in practice.

Provided an abstraction to temporarily add or remove applications as
several tests messed with app_config.installed to achieve this effect.
For now this API is _-prefixed because it looks dangerous.

It wasn't used anywhere and couldn't be implemented any more since
non-installed apps no longer have a configuration.

Got rid of AppConfig._stub. As a side effect, app_cache.app_configs now
only contains entries for applications that are in INSTALLED_APPS, which
is a good thing and will allow dramatic simplifications (which I will
perform in the next commit). That required adjusting all methods that
iterate on app_configs without checking the "installed" flag, hence the
large changes in get_model[s].

Introduced AppCache.all_models to store models:
- while the app cache is being populated and a suitable app config
  object to register models isn't available yet;
- for applications that aren't in INSTALLED_APPS since they don't have
  an app config any longer.

Replaced get_model(seed_cache=False) by registered_model() which can be
kept simple and safe to call at any time, and removed the seed_cache
argument to get_model[s]. There's no replacement for that private API.

Allowed non-master app caches to go through populate() as it is now
safe to do so. They were introduced in 1.7 so backwards compatibility
isn't a concern as long as the migrations framework keeps working.

Thanks Tim Graham for review.

This reverts commit a25fe3b6.

It didn't do anything. It merely added to every PO file:

msgid "%(name)s"
msgstr "%(name)s"

Thanks Ramiro Morales for the report at:
https://groups.google.com/d/msg/Django-I18N/vc2vQzv80UQ/EuaW38V7X7sJ

Conflicts:
	django/contrib/admin/templates/admin/index.html

Refs #19031.

It's now consistent with RenderContext.get.

Thanks graeme.perrow at sap.com for the original report.

Thanks Bruno Brouard for the report.

* Removed ADMIN_FOR setting and warn warning
* Group view functions by namespace instead of site
* Added a test verifying namespaces are listed

Thanks to Claude Paroz for reviewing and ideas for improvement.

Callable arguments were an untested and undocumented feature.

Using callables as query parameters is undocumented and not working,
so this changes an example from the ForeignKey.limit_choices_to
documentation that uses it.

Thanks Anubhav Joshi for the documentation.

Used the information from the app cache instead of creating a duplicate
based on INSTALLED_APPS.

Model._meta.installed is no longer writable. It was a rather sketchy way
to alter private internals anyway.

Improved Andrew's hack to create temporary app caches to handle
migrations. Now the main app cache has a "master" flag set to True
(which is a non-default keyword argument, thus unlikely to be used by
mistake). Other app cache instances have "master" set to False.

The only sanctioned way to access the app cache is by importing
django.core.apps.app_cache.

If you were instanciating an app cache and relying on the Borg pattern,
you'll have to refactor your code.

Added comments in the three empty models.py files that are still needed.

Adjusted the test runner to add applications corresponding to test
labels to INSTALLED_APPS even when they don't have a models module.

Thanks Marc Tamlyn for the suggestion.