Unused since 5d863f1f.

ModelAdmin added readonly_fields to exclude, but would not undeclare
them if they were overridden.

Thanks Bojan Mihelac for the initial patch.

Setup QUnit, added tests, and measured test coverage.

Thanks to Nick Sanford for the initial tests.

On sqlite the SUM() of a decimal column doesn't have a NUMERIC type so
when comparing it to a string literal (which a Decimal gets converted to
in Django) it is not compared as expected.

The password from settings.py is written in a temporary .pgpass file
file whose name is given to psql using the PGPASSFILE environment
variable.

Thanks Tim Graham for review.

Made it possible to register and run callbacks after a database
transaction is committed with the `transaction.on_commit()` function.

This patch is heavily based on Carl Meyers django-transaction-hooks
<https://django-transaction-hooks.readthedocs.org/>. Thanks to
Aymeric Augustin, Carl Meyer, and Tim Graham for review and feedback.

This reverts commit bd691f45 (refs #24177).

If the only manager on the model is the default manager defined
by Django (`objects = models.Manager()`), this manager will not
be added to the model state. If it is custom, it needs to be
passed to the model state.