Fixed #23975 -- Restored pre_migrate signal if all apps have migrations.

Thanks kmmbvnr for the report.
Fixed the formatting of one section of the security page
Fixed #24002 -- GenericRelation filtering targets related model's pk

Previously Publisher.objects.filter(book=val) would target
book.object_id if book is a GenericRelation. This is inconsistent to
filtering over reverse foreign key relations, where the target is the
related model's primary key.
Fixed display of lists after website redesign

Thanks Brian Jacobel for the report. refs django/djangoproject.com#197
Fixed #23497 -- Made admin system checks run for custom AdminSites.
Removed netCDF from GIS driver testing; refs #23804.

It may not be installed on all systems.
Fixed #24007 -- Ensure apps registry's ready before unpickling models

This prevents AppRegistryNotReady errors when unpickling Django
models from an external script.
Simplified handling of a default value.
Renamed get_template_from_string to from_string.

The shorter name is just as explicit and, well, shorter.
Cleaned up the django.template namespace.

Since this package is going to hold both the implementation of the Django
Template Language and the infrastructure for Multiple Template Engines,
it should be untied from the DTL as much as possible within our
backwards-compatibility policy.

Only public APIs (i.e. APIs mentioned in the documentation) were left.
Imported BaseEngine from the DEP.

i18n is left aside for now.
Added initial support for loading template engines.
Added dummy template backend.
Added jinja2 template backend.
Added Django template backend.
Added basic tests for template backends.
Removed some uses of global APIs from django.template.loader.
Passed a reference to the current engine when instanciating Template.
Isolated template tests from Django settings.
Looked up the default template engine in the list of all engines.