Enable subproject filters for projects with subprojects (#11674)
* Enable subproject filters for projects with subprojects
This at least shows the UI element if it makes sense to show.
Currently there's no way without knowing the query syntax to get subprojects,
which has caused confusion for a few of our users and broke existing behavior.
Refs https://github.com/readthedocs/readthedocs.org/pull/11638
* Also show on subprojects
* Update to support subprojects having the right filter
* Fix tests
* Apply suggestions from code review
Co-authored-by: Manuel Kaufmann <humitos@gmail.com>
* Check for version
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Co-authored-by: Manuel Kaufmann <humitos@gmail.com>