WIP: Allow Thumbnail Choice Blocks To Use All Icons In A Directory

[dchukhin]

One-line summary

This pull request updates our IconChoiceBlock to use all icons in a directory.

Significant changes and points to review

More specifically, the changes are:

• the most recent changes in wagtail_thumbnail_choice_block support pointing a ThumbnailChoiceBlock at a directory, and Wagtail users being able to choose any of the icons from that directory.
• IconListItemBlock now uses a ThumbnailChoiceBlock, setting the thumbnail_directory to img/firefox/flare/2026/icons, which makes all icons in that directory (and all of its subdirectories) choices for the block
• springfield may require CSS classes for the icons in the directory, so a new springfield/cms/management/commands/generate_flare_icon_css.py management command allows creating a file of CSS rules, setting the --icon-src for each of the files in the directory (and all of its subdirectories). I ran this command for the files in media/img/firefox/flare/2026/icons/desktop-16, and added CSS rules for them to media/css/cms/flare26-icon.css.
• some helper function have been added in a springfield/cms/icon_utils.py file. For example, icon_value_fn() takes a relative path to a file, and returns an icon name (given "desktop-16/arrows-and-chevrons/forward-16", it returns "forward"). This function is used by the ThumbnailChoiceBlock to determine which value to save in the database when a user chooses an icon. Since a number of icon paths would result in identical values ("mobile-24/arrows-chevrons/forward-24.svg" would also want to be "forward"), any value collisions are handled explicitly in the icon_value_fn.

TODO:

• improve styling for the block in Wagtail

Issue / Bugzilla link

Testing

1. install the most recent (unreleased) wagtail_thumbnail_choice_block changes: pip uninstall wagtail-thumbnail-choice-block and pip install wagtail-thumbnail-choice-block@git+https://github.com/lincolnloop/wagtail-thumbnail-choice-block.git@main
2. run the migrations on this branch
3. go through the pages that already use the icon choice block, like what's new pages, and make sure that the icons show up in the template, and Wagtail users can change them successfully
4. find an article detail page that has an icon set, and make sure that it is still set and shows up as expected in article cards list blocks
5. try to create a new page with icon choice blocs, and verify that the icons show up as expected
6. create a new page that uses an article cards list block. For example, create a free form 2026 page, add a section to the content, and add an article cards list block to the section, and choose an article and set the card type to be "Icon Card". Verify that the article detail page's icon shows up in the article card on the new free form 2026 page.