Explicit exception chaining for db exceptions by setting __cause__ in py2

Use __cause__ to expose the underlying database exceptions even
under python 2.

Auth views should be reversed by name, not their locations in
`django.contrib.auth.views`. This allows substituting your own
implementations of the auth views.

Thanks wim@ for the suggestion.

A test failed if the path to the Django checkout contained a dot.

Refs #20485.

Thanks Anssi for reporting this performance bottleneck.

This method is useful to discover files inside apps.

Also regenerated the affected translation template.

Refs #20540.

Thanks d.willy.c.c@ for the suggestion and Melevir and Areski Belaid for work
on the patch.

Thanks lborgav for the initial patch.

Thanks i.amber.jain@ for the report.

Thanks KyleMac for the report, André Cruz for the initial patch and
Hiroki Kiyohara for the tests.

The regression was caused by patch to ticket #15316 and was fixed by a
patch to #10790.

It's useful to be able to list all the (flattened) keys of a
ContextList, to help you figure out why the variable that's supposed
to be there is not.

No .values() or .items() added as the definition for those aren't clear.

The patch is Chris Wilson's patch from pull request 1065 with some
modifications by committer.

Fixed #18681 -- BaseModelAdmin.get_form and InlineModelAdmin.get_formset no longer bypass get_fieldsets

Thanks msopacua for the report.

Thanks czambran.

Thanks tris@ for the report.

Thanks wim@ for the suggestion.

Thanks tinodb for the report.

Fixed #20531 -- Don't hard-code class names when calling static methods

Oracle doesn't like grouping by TextField, so use CharFields instead in
models.

Thanks Chris Wilson for the report and initial patch.

Thanks dave@ for the suggestion and Rohan Jain for the patch.

Clearer explanation when exception has no message

Thanks Carl for the suggestion and David Cramer for the patch.

Thanks Gavin Wahl.

Thanks weipin for the report and Baptiste Mispelon for the patch.

Correctly calculate the ``aggregate_start`` offset from loaded fields,
if any are deferred, instead of ``self.query.select`` which includes all
fields on the model.

Also made some PEP 8 fixes.