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.

Fixed #11078 -- documentation update.

Without that, the Note model would be initially created and then
the tests using that model failed when run in isolation.

The only problem for Oracle was the test, which tested nullity on
text/char fields -- but Oracle interprets_empty_strings_as_null.

Obsolete since refs #23867.