Fixed #24706 -- Made ModelForm._post_clean() handle a ValidationError raised when constructing the model instance.

Thanks Loïc Bistuer for review and advice.

Use the same code path for:

- a database that has the same settings as another database
  (as defined by test_db_signature)
- a database this is defined as a mirror of another database

There's no conceptual difference between these two cases.

Thanks Shai for the suggestion.

The return value of setup_databases is only used as an argument for
teardown_databases which doesn't need mirrors.

In order to reuse another database, Django must use set_as_test_mirror.

It contained two sections separated by comments. It makes more sense to
put each section in its own function.

They were removed in Django 1.9.

I could leave the reference to TEST_DEPENDENCIES in the 1.2.4 release
notes because the link points to the right location and the name was
accurate at the time.

Added the CSRF_TRUSTED_ORIGINS setting which contains a list of other
domains that are included during the CSRF Referer header verification
for secure (HTTPS) requests.

Thanks Anssi Kääriäinen for providing the solution.

The corresponding HTML was removed in
738d9af1.

Thanks Marcin Biernat for the initial patch and tests.

Fixed #25058 -- Added GenericRelations with related_query_name to the admin's delete confirmation page.