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 #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.
Fixed #22659 -- Prevent model states from sharing field instances.

Thanks to Trac alias tbartelmess for the report and the test project.
Fixed #22711 -- Adjusted ordering checks to allow implicit relation fields.

refs #19195.
Fixed #22744 -- Fixed sqlite3 get_relations introspection with views

Thanks Tim Graham for the report and Simon Charette for the review.
Revert "Fixed #19303 -- Fixed ModelAdmin.formfield_overrides on fields with choices"

This reverts commit 9d1987d7679165ad3a7c2b713a8a488cc1421905.
Fixed #22690 -- Added a check for proxy models containing fields.

Removed the FieldError raised by ModelBase.__new__ in this case.
Fixed #22748 -- Corrected post_migrate usage example.

Thanks Rudolph for the report.
Fixed #11776 -- Added CSS class for non-field/top of form errors.

Thanks Daniel Pope for the suggestion.
Fixed #22753 -- Documented ability to use a note with the {% comment %} tag.