Subcategory carousels on category views

For category views, we could use a more interesting layout to aide with discovery. One idea I had is to pull out subcategories ([FD.o "Additional Categories"](https://standards.freedesktop.org/menu-spec/latest/apas02.html)) into a carousel (like "Trending" and "Recently Updated" on the homepage). Eventually these could link to a subcategory view with all of the matching apps.

We will likely want to determine how many apps in a subcategory cause the subcategory to show up in the first place.

Rough whiteboard sketch: