Fixed #21870 -- Admin check for list_editable_item

During the admin check for list_editable _check_list_editable_item
should return an empty list if all checks pass. Additionally the
Testcase test_readonly_and_editable was changed to test what the
name implies instead of duplicating the logic of test_readonly.
Fixed typo in error message.
Fixed #21867 -- Removed AppStaticStorage; app paths are now AppConfig's job.

AppStaticStorage only provided one thing over FileSystemStorage, which was
taking an app name (import path) and translating it into a filesystem
path. This is now something that should be done via app_config.path instead,
leaving AppStaticStorage with no reason for existence. It should be safe to
remove, as it was undocumented internal API.

There was some kind of feature in the AppDirectoriesFinder code related to a
"prefix" attribute on the storage class used by AppDirectoriesFinder. Since
this feature was undocumented, untested, and of unclear purpose, I removed it
as well.
Fixed #21783: More SQLite default fun with nulls.
Unroll choices iterators in field deconstruction
Fixed #21871 -- Fixed Apps.is_installed() for apps with custom label.

Thanks Aymeric for design discussion.
Fixed #21829 -- Added default AppConfigs.

Thanks Russell for the report, Marc for the initial patch, Carl for the
final review, and everyone who contributed to the design discussion.
Merge pull request #2211 from carljm/t21867

Fixed #21867 -- Removed AppStaticStorage; app paths are now AppConfig's job.
Added ticket #14007 contributors to AUTHORS
Fixed a failing schema assertion.

BooleanFields are stored as TINYINT(1) on MySQL.
Merge pull request #2213 from dnerdy/ticket-14007-contributors

Added ticket #14007 contributors to AUTHORS
Fixed #21836 -- Improved transaction docs about autocommit mode

Clarified that queries in autocommit mode are committed immediately
only if a transaction has not already been started. Added to the
main transaction docs that Django's TestCase class implicitly wraps
its tests in transactions.
Fixed #21873 -- Removed duplicate import.

Thanks Sephi for the report.
Fixed #17304 -- Allow single-path and configured-path namespace packages as apps.

Also document the conditions under which a namespace package may or may not be
a Django app, and raise a clearer error message in those cases where it may not
be.

Thanks Aymeric for review and consultation.
Docs correction: apps loaded from eggs do not have appconfig.path None.
Moved sys.path-extending decorator to django.test.utils and used throughout test suite.

Thanks Aymeric for the suggestion.
Moved RequestSite and get_current_site.

Following the app-loading refactor, these objects must live outside of
django.contrib.sites.models because they must be available without
importing the django.contrib.sites.models module when
django.contrib.sites isn't installed.

Refs #21680. Thanks Carl and Loic for reporting this issue.
Fixed typo. Refs #21836.
Fixed #21702 -- get_model('app_label.ModelName').

Also added tests for get_model.
Took advantage of the new get_model API. Refs #21702.