Refactored ValidationError to allow persisting error params and error codes as the exception bubbles up

Thanks to frog32 for the report and to Tim Graham for the review.

There were a couple of places which used Query.join() directly. By
using setup_joins() in these places the code is more DRY, and in
addition there is no need to directly call field.get_joining_columns()
unless the field is the given join_field from get_path_info(). This
makes it easier to make sure a ForeignObject subclass generates joins
correctly in all cases.

Thanks batisteo for the report.

Refs #20599.

The _load_library method on BasePasswordHasher turns ImportErrors
into ValueErrors, this masks ImportErrors in the algorithm library.
Changed it to a clearer worded error message that includes
the ImportError string.

Thanks Chris Lasher for the report.

By avoiding to run syncdb with the full set of test models.

Thanks Anssi for the idea.

Fixed #20565 -- Added template examples for GCBV.

Fixed #20598 -- Add new HTTP status codes defined in rfc6585

Thanks to David Reitter for the report,
and to Marc Tamlyn and Baptiste Mispelon for the review.

The join used by select_related was incorrectly INNER when the query
had an ORed filter for nullable join that was trimmed away. Fixed this
by forcing the join type to LOUTER even when a join was trimmed away
in ORed queries.

428, 429, 431 and 511

Thanks to @DrMeers for the report and @ludwigkraatz for the initial patch.

Remove incorrect CONTENT_TYPE header from GET and HEAD requests

Refs #20532

Thanks carbonXT for the report.

Fixed order of base classes in CBV mixin docs

Older versions of uWSGI and Sentry's middleware do not adhere to
the WSGI spec and cause the `request_finished` signal to never
fire. Added notes to the appropriate places in the docs.

Fixed #20537.

Also moved its documentation to the 'advanced' section. It doesn't
belong to the 'overview'. Same for TransactionTestCase.reset_sequences.

When available_apps is set, after a TransactionTestCase, the database
is now totally empty. post_syncdb is fired at the beginning of the next
TransactionTestCase.

Refs #20483.

Simplified a formset example in the docs

Fixed #20591: added version in docs for utils.module_loading.import_by_path

This reverts commit 64041f0e.

lookup.tests.LookupTests.test_regex_non_string fails under Postgres.
We should also try to rewrite the test using an existing model.