Django-cast

Use the original image if no renditions are generated. Includes now a
management command to sync renditions. Revisited how the renditions are
passed to the template.

- 112 If no renditions are generated, use the original image as the default rendition
- 113 Management command to sync renditions
- Changed tox.ini to include Django 5.0 and removed Django 4.0

Responsive images using the `picture` element and some smaller fixes.

- Make title link to detail and strip dash from date link in bootstrap4
template thanks neingeist
- Re-added the `o` ordering parameter to make it possible to order by
visible date
- Upgraded Python Version for GitHub Actions to 3.12
- 108 Fixed test coverage for `cast/filters.py`
- 109 Responsive images revisited
- picture element and avif format for images
- images now have an `alt` attribute
- using `srcset` in combination with `sizes` attributes for smaller images
- removed `wagtail_srcset` dependency
- 110 Fixed mypy errors by putting a workaround in place

Added Python 3.12 support, dropped Python 3.9 support.

* Workaround for missing comment form error messages 107 (thanks petermeissner)
* Python Version support (+3.12 -3.9) 106
* A little bit more documentation for database restore 56

Resolved a significant bug in theme selection. If you picked a theme
stored in your session, the system would mistakenly apply a pre-selected
theme for HTML fragments rendered through the JSON API. This was due
to the real theme choice not being correctly passed from the JSON API
to the Wagtail page, resulting in a completely dysfunctional Vue theme.

* Bugfix theme selection 105
* Fixed mypy issues by django-stubs update + one small fix 101
* Improved documentation for theme selection 105
* Got rid of ProxyRequest in favor of a simple HtmlField 105
* Fixed searching for name instead of slug when filtering tags 100
* Added a `has_selectable_themes` flag to the context of blog pages to make it easy to decide whether a theme selector can be showed 105

Make it possible to select a new theme on the website and store the selection result in the session.

- Added a snippet viewset to be able to do CRUD on tags via wagtail admin 100
- Let users select a theme for the site and store the choice in the session 105
- Added a new api endpoint to get the list of available themes and update the selected theme 105

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