Thanks to Tim Graham for the review.

Forwardport of ae1d663b
from stable/1.8.x plus more.

Thanks Peter Schmidt for the report and the initial patch.
Thanks to ​Oktay Sancak for writing the original failing test and
Alvin Savoy for supporting contributing back to the community.

Thanks Carl Meyer for the suggestion and Alex Gaynor and Carl for reviews.

This fixes redirecting to relative URLs with django.shortcuts.redirect.

This (nearly) completes the work to isolate all the test modules from
each other. This is now more important as importing models from another
module will case PendingDeprecationWarnings if those modules are not in
INSTALLED_APPS. The only remaining obvious dependencies are:

- d.c.auth depends on d.c.admin (because of the is_admin flag to some
  views), but this is not so important and d.c.admin is in
  always_installed_apps
- test_client_regress depends on test_client. Eventually these should
  become a single module, as the split serves no useful purpose.

* Removed trailing whitespace.
* Added newline to EOF if missing.
* Removed blank lines at EOF.
* Removed some stray tabs.

Refs #20680.