This is a performance optimization and also fixes test errors with the
upcoming merge of contrib tests into tests/. The tests failed on MySQL
because the models with GeometryField were being checked but the
non-GIS MySQL backend didn't know how to handle them.

Removed fragile algorithm to find which application a model belongs to.

Fixed #21680, #21719. Refs #21794.

During the autumn DST change, the template engine would fail to convert
the naive representation of now when USE_TZ = True. Passing now in UTC
eliminates the issue.

Thanks mbertheau and tricoder42.

Refs #23714.

This reduces the length of rope RequestContext gives users to hang
themselves with.

Thanks Alex Hill for the report and Tim Graham for the review.

Used setUpTestData, client login/logout and manager's count() method.

Thanks to Tim for the review.

Thanks to Tim for the review.

Numpy will be used in both the geos and gdal modules,
so the import should sit in the parent module gis.

This significantly improves performance on PyPy. The previous
implementation would generate a new class on every single request,
which is relatively slow.

This dramatically improves performance on PyPy. The following benchmark:

python -mtimeit -s "from django.utils.functional import allow_lazy; from django.utils.translation import ugettext_lazy; f = allow_lazy(lambda s: s, str)" "f(ugettext_lazy('abc'))"

goes from 390us per loop to 165us.

Refs #15779. This will allow easier admin URL changes, when needed.
Thanks Simon Charette for the review.

Report and original patch by Kay Cha.

Revert "Fixed #24075 -- Prevented running post_migrate signals when unapplying initial migrations of contenttypes and auth"

This reverts commit 737d2492.

This reverts commit 51dc617b.