Changed internal storing of abstract and concrete managers to be in a single list.

This commit prepares the internal manager layout to be serialized by
migrations; refs #23822.
Fixed #23822 -- Added support for serializing model managers in migration

Thanks to Shai Berger, Loïc Bistuer, Simon Charette, Andrew Godwin,
Tim Graham, Carl Meyer, and others for their review and input.
Fixed #23884 -- Moved FlatPageSitemap into django.contrib.flatpages.
Moved an import in an auth test; refs #23925.

This keeps tests/__init__.py from importing other modules and may fix a problem
with test discovery revealed in formtools tests on Travis CI.
Refs #23947 -- Fixed admin_views test execution order dependency.
Corrected test assertion from refs #21403.
Refs #23947 -- Fixed queries test execution order dependency.

Specify an id to avoid conflict with objects created in setUpTestData.
Fixed #23987 -- Made SQLite SchemaEditor always use effective_default().
Fixed typos in docs/releases/1.7.2.txt.
Fixed #23405 -- Fixed makemigrations prompt when adding Text/CharField.

A default is no longer required.
Fixed reverse test execution for migration manager tests

Thanks to Tim Graham for reporting the issue; refs #23822
Fixed refs #23987 test on Oracle.
Refs #23947 -- Improved migrations tests table cleanup.

Copied technique from schema tests.
Refs #23884 -- Silenced a deprecation warning in sitemaps tests.