Editable Map Pool

[shenanigans-be]

Suggested by MrDrBojangles on Reddit:

Be able to make a selection of tiles that are then shuffled into valid slices.

Thoughts:

• Since "take selected tiles and make slices" is already a part of the process this should be pretty straightforward to implement
• "Invalid" selections will happen more, so we'd have to watch out for that and return some nice and informative error message back to the user.