Django-cast

Smaller fixes and improvements around tags, categories and filters.

* Fixed the .readthedocs.yml 102
* Moved facet filter related logic into the facet filter classes, this is a lot cleaner 100
* Remove empty facet filter fields from filterset form 100
* Refactored the post filter classes to be more consistent 100
* Category and tags are now available for podcast episodes, too 100
* Facet counts for tags and categories are now available via the API 100

Added tags and categories for posts. I planned for long adding a feature like this, but avoided implementing it because I didn't understand what I did in `filters.py`. It's a beta feature and I probably will keep only categories or tags, but not both.

* Updated some documentation in top level README.md
* Added tags / categories to posts 100 😅
* Temporarily deactivated the mypy pre-commit hook because mypy 1.5.0 is not compatible with django-stubs
* Had to drop support for Wagtail 3 because of conflicting migrations (taggit)

Use the STORAGES setting introduced in Django 4.2 to improve media backup and restore 99.

Improved plain theme.

- Better pagination styling for plain theme.
- Update pre-commit hooks
- Include wagtail 5.1 in test matrix (tox.ini)
- Fixed the "land on last page by default" wagtail admin pagination bug 98
- Fixed the broken chooser buttons for audio and video (wagtail 5.1, action-chooser class was removed from button) 98
- Fixed duplicate chooser button for audio and video 98
- Added some documentation for audio content

Finished support for SPA themes (comments and podlove player).

- Made comments data available via wagtail api 95
- Made some data available just via page attribute, because it's only added via template and script tag 95

Support for SPA themes and some htmx fixes.

- Expose data via template context to theme (for vue theme)
- pagination page size
- wagtail api page url via `reverse("cast:api:wagtail:pages:listing")`
- facet count api base url
- Add overview_html and detail_html to the Post model to get the rendered html in the vue theme
- The template for image galleries can now be overwritten by themes
- Fixed audio player on htmx pagination
- Fixed galleries on htmx pagination
- Don't remove newlines in *_html because it breaks preformatted code blocks
- Combine wagtail api pages endpoint with django-filter to allow filtering by date facets and fulltext search
- Add facet count api endpoints