Update TODO now that initial round selection is available

One TODO item is clearly complete here: The new multi-select field in
the modal form when creating a puzzle. That's working well enough that
I don't think we need the separate /round command so I'm dropping that
from the TODO.

Two little things are needed to really make round support complete:

  * Display/grouping of rounds in the Turbot home view

  * Select same rounds (by default) when creating a new puzzle in the
    same channel of an existing channel