Refs #22015

Refs #21911.

It's now easier to achieve the same effect with modify_settings or
override_settings.

Reverted 4a56a93c.

Since applications that aren't installed no longer have an application
configuration, it is now always True in practice.

Provided an abstraction to temporarily add or remove applications as
several tests messed with app_config.installed to achieve this effect.
For now this API is _-prefixed because it looks dangerous.

* Removed ADMIN_FOR setting and warn warning
* Group view functions by namespace instead of site
* Added a test verifying namespaces are listed

Thanks to Claude Paroz for reviewing and ideas for improvement.

Thanks Bouke Haarsma for the review.

Don't set a global default interpreted role function for reStructuredText.
Instead, use the `default-role` directive to change the default only within
the `parse_rst()` function.

Thanks Malcolm Tredinnick for the report.