fix: show correct icons in the sidebar for units with custom XBlocks

[Agrendalath]

Currently, the sidebar relies only on the XBlock's category class attribute (called type in the transformers). This behavior is inconsistent with the legacy subsection navigation, which relies on the XModuleMixin.get_icon_class method. This commit adds the icon_class to the fields collected by the transformers and uses it to determine whether the "problem" or "video" icon should be displayed for a unit in the sidebar.

Testing instructions

1. Ensure you have the following Waflle flags (/admin/waffle/flag/) set to "Yes":
   1. courseware.enable_navigation_sidebar
   2. courseware.disable_navigation_sidebar_blocks_caching
2. Create a new unit with a single HTML block and publish it.
3. Go to the live version and see that the sidebar icon for this unit is fa-book.
4. Add an icon_class = 'video' field to the HtmlBlockMixin class (we could use a custom XBlock here, but this is a faster way to test this).
5. Make any changes to the block you previously created and publish them.
6. Check the live version again and see that the unit icon is now fa-video.
7. Add a Problem block to the same unit you've used before and publish it.
8. Check the live version again and see that the unit icon is now fa-edit.

Private-ref: BB-9332

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