Better block layout for shared flappy puzzles

We want a nice two column layout for Flappy puzzles when you click ‘How it Works’.  This allows you to specific a block arrangement on the app’s config.  The block arrangement is a mapping of block type to x/y location.  The order of precedence for the arranger is:
(1) If block xml already has an x/y, use that
(2) Otherwise, if block appears in the arrangement mapping, use x/y from that.
(3) Use the same arrangement logic as before, where blocks are spaced vertically in a single column.