[Feature] Support more complex category/game selection

[dustin-jw]

Feature Request

Description

Rather than a flat list of links to question categories, we could reorganize them so that on the home page, you choose a format, like "Flash Cards" or "Multiple Choice". Then on the next page, you can choose one or more categories (or all), then start the game.

This will take some reorganizing, and supporting multiple categories will take a bit of design work and depend on and expand the work from #22.