Add simplified "Set Jr" mode with only solid cards

[robot-dreams]

This PR is an attempt to address #110 and builds on the existing work in #121 (as well as the discussion included there).

The simplified "Set Jr" game mode involves a 27 card deck (instead of the usual 81 card deck). However, the deck for a particular game is generated at game creation time (in particular, before the final mode has been selected). I tried to address this by regenerating a deck in the frontend every time the user selects a new mode, but admittedly this is a pretty hacky approach and I'm open to other suggestions.

Unlike #121, this change keeps the 12 card board size, in order to reduce the scope of the changes needed.

I tried to account for all places that needed updating by searching for "normal" (another game mode), but since I'm new to the codebase it's very possible I missed something.