Deborgified the app cache.

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.
Normalized Model._meta.installed.

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.
Removed forgotten reference to the borg pattern.
Style cleanup forgotten in second previous commit.
Fixed #21625 -- incorrect documentation for DiscoverRunner.build_suite
Refs #17413 -- Added isinstance backward compatibility on ErrorList.
Fixed #21552 -- Allowed the use of None for the iexact lookup.

Thanks Anubhav Joshi for the documentation.
Fixed #21632 -- Docs: Removed example with callable as query parameter

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.
Fixed #11629 -- Deprecated callable arguments to queryset methods.

Callable arguments were an untested and undocumented feature.
Fixed #21386 -- Removed admindocs dependence on sites framework

* 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.
Simplied some code in django.conf.__init__.
Fixed #21613 -- Removed a hardcoded input id from the tutorial.

Thanks Bruno Brouard for the report.
Fixed #21585 -- Updated some links to 3rd party database projects.

Thanks graeme.perrow at sap.com for the original report.
Fixed #21639 -- Implemented RenderContext.__getitem__

It's now consistent with RenderContext.get.
Small flake8 fixes -- number of blank lines between top level definitions
Set stacklevel for the override_settings warning.

Refs #19031.
Revert "Changed admin/templates/admin/index.html to make <caption> translatable"

This reverts commit a25fe3b65e39ff6f59ac545d7934461be6c13612.

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