Thanks Carl Meyer for review.

Thanks datakid for the report

Refs #15053.

This now works for multiple Django engines and recursive loaders.
Support for non-Django engines is still pending.

Refs #15053.

Thanks Simon Charette and Tim Graham for the reviews.

Thanks Serhiy Zahoriya for the suggestion and Tim Graham for the
review.

Thanks tttomekkk for the report.

Refs #24354 -- Prevented repointing of relations on superclasses when migrating a subclass's name change

The issue was hidden on 1.8+ until #24573 due to a bug inside the model
reloading process.

Forwardport of patch from ae87ad00

Changed ModelState.clone() to create a shallow copy of self.fields
and self.managers.

Thanks to Elmar Bucher for the report and Tim for the review.

Thanks to knbk for the report.

Created bulk_update() context manager on StateApps. Sped up unregistering
models in reload_models() by using this context mananger.

Thanks Carsten Fuchs.

This continues work to treat Django templates as a library.