Fixed #22684 -- Added `empty_label` option on `django.forms.extras.widets.SelectDateWidget`

Thanks danielsamuels for the report
Fixed constant_time_compare on Python 2.7.7

Python 2.7.7 includes compare_digest in the hmac module, but it requires
both arguments to have the same type. This is usually not a problem on
Python 3 since everything is text, but we have mixed unicode and str on
Python 2 -- hence make sure everything is bytes before feeding it into
compare_digest.
Set straight the sense of 'uppercases_column_names' feature flag
Fixed #22720 -- Migrations attempt to create _order twice.
Adding test to fix of duplicate _order fields (#22720)
Merge pull request #2734 from valberg/double_order_trouble

Fixed #22720 -- Migrations attempt to create _order twice.
Simplified, very slightly, some code in the oracle backend
Fixed several typos in Django
Fixed a typo that Tim Graham noticed
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.