Fixed #22844: Duplicate SQL for SQLite FKs
Removed usaged of contrib.sessions as a placeholder in migration tests.

Without this, we're unable to add actual migrations for the app.
Added database migration for contrib.sessions.

refs #22170.
Fixed #22788 -- Ensured custom migration operations can be written.

This inspects the migration operation, and if it is not in the
django.db.migrations module, it adds the relevant imports to the
migration writer and uses the correct class name.
Use __first__ not __latest__ for unknown app dependencies
Fixed #22848: Ignore no-migrations errors during makemigrations only
Fixed #22783: Make sure swappable models come first in creation
Fixed #22847: Optimizer wasn't expecting unresolved FKs
Added database migration for contrib.contenttypes.

Moved contenttypes tests to allow them to run correctly in the presence of
migrations. refs #22170.
Fixed flake8 error.
Added database migration for contrib.auth.

refs #22170.
Fixed #22845 -- Correctly handled memcached default timeout value.
Ignore through being None on some special cases of fields.
Ignore more blank throughs on swapped-out M2Ms
Removed some u'' prefixes to fix Python 3.2.
Renamed DatabaseFeature.supports_check_constraints to supports_column_check_constraints.

Thanks maxi for the suggestion.