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.
Updated TemplateResponseMixin docs to reflect new behavior after refs #23789.
Allowed Python executable names other than python in test from refs #24007.
Fixed #24014 -- Unified OGRException and GDALException

Thanks Tim Graham for the review.
Added RasterSource/GDALBand GDAL objects

Based on Daniel Wiesmann's raster branch. Thanks Daniel Wiesmann
and Tim Graham for the reviews. Refs #23804.