Fixed #21513 -- Added method_decorator support for argumented decorator

Copied attributes into the decorated method and special case __name__
copy as this will not be present on a Class object. Added regression
test to decorator suite.
Remove check_migrations from the runserver command and use the new checks framework to check for unapplied migrations. Don't check for migrations if the DATABASES setting is empty.
Adding tests for check_migrations.
Replace _parse_content_type with cgi.parse_header
Merge pull request #1997 from dpwrussell/method_decorator_args_fix

Used available_attrs in method_decorator
Merge pull request #2244 from mlavin/21856-migration-checks

Fixed #21856: Allow Empty DATABASES Setting
Fixed #21892: RunPython no longer accepts strings
Fixed inaccuracies in generic mixins documentation.
Fixed #21917: Overly cautious SQLite3 backend for null fields + defaults
Removed unused imports + other flake8 fixes.
Fixed #21954: Raise nice error when serializing datetimes with timezones
Fixed #21247 -- Made method_decorator play nicely with descriptors

When a method decorator was used in conjunction with a decorator
implemented as a descriptor, method_decorator did not correctly respect
the method binding.

Thanks for Graham Dumpleton for the report and initial patch.
Fixed #21967: Added check for object in ModelFormMixin.get_form_kwargs.

Thanks lagovas.lagovas at gmail.com for the report.
Fixed #21968: Bad detection of old-style apps to add initial migration
Raise (pending) deprecation warning for allow_syncdb.
Fixed #21969: Fix behaviour of initial_data with migrated apps
Fixed #21842: Remove redundant DatabaseFeatures.max_index_name_length
Fixed #21482 -- Uplifted restriction of collectstatic using symlink option in Windows NT 6.

Original patch by Vajrasky Kok. Reviewed by Florian Apolloner, Aymeric Augustin.
Fixed #21844: Move quote_parameter off of Operations and rename
Fixed #21958: Handle dependencies for swappable models in AddField