Fixed #21774 -- Isolate all test urls from eachother.

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.
Fixed typo in comment.
Fix typo in storage docs
Merge pull request #2171 from kwood/master

Fixed typo in storage docs
Merge pull request #2170 from numerodix/master

Fixed typo in comment.
Fixed #21747 -- Added {% elif %} to template topics guide.

Thanks gcc for the suggestion.
Fixed #21727 -- Added some links to help clarify topics/auth/default.txt

Thanks daGrevis for the suggestion.
Fixed #21768 -- Corrected TemplateView context section.

Thanks nedbatchelder for the report and claudep for the patch.
Fixed #21343 -- Noted many-to-many restrictions with intermediate model.

Thanks EvilDMP for the report.
Fixed #21740 -- Stopped using mutable default arguments in test client

Thanks Denver Coneybeare for the report and initial patch, and
Atala for another patch.
Fixed #21784: Correctly decode stdin input for migration defaults