Fixed #20550 -- Added ability to preserve test db between runs
Implement Migration.run_before

This attribute (used for reverse dependencies) was previously declared
and mentioned in the code, but never actually used.
Fixed #20401 -- ContentTypeManager.get_for_model reads from db_for_read.

Thanks Simon Charette and Tim Graham for the reviews.
Making SQL management commands migration aware.
Merge pull request #2743 from valberg/migration_aware_sql_commands

Making SQL management commands migration aware.
Fixed #22682 -- `makemigrations` will create `MIGRATION_MODULES` package

`makemigrations` will automatically create the package specified
in `MIGRATION_MODULES` if it doesn't already exist.

Thanks ovidiuc4 for the report.
Fix test failure caused by 3149cdce32e7261b2d44128ab712c4d4f1a5d429
Merge pull request #2744 from mmardini/MIGRATION_MODULES

Fixed #22682 -- `makemigrations` will create `MIGRATION_MODULES` package
Improved deprecation plan links in release notes.
Fix additional test failures caused by migration pollution
Fixed #22710 -- Documented aggregation behavior for empty QuerySets.

Thanks matklad for the report.
Improved the test for refs #22682.

Thanks apollo13 for the review.
Fixed #19905 -- Fixed leakage of file descriptors in form wizard.
Removed an unused import
Note that the stdlib's version of pbkdf2_hmac will also be used in the upcoming Python 2.7.8 release
Revert "Making SQL management commands migration aware."

This reverts commit cb9c9a7b5879671053c7d2ad6e79943a8814b274.
Merge pull request #2726 from gchp/ticket-20550

Fixed #20550 -- Added ability to preserve test db between runs
Split GenericRelationsTests.test_generic_relations into several tests; refs #18586.
Fixed #21938 -- Moved documentation for error views to reference guide.
Fixed syntax error in ogrinspect.py management command.