make "disabled" dice unclickable, and put unclickable dice on a grey background

Attempt to make things a little bit friendlier for players by indicating visually which dice can't be clicked.  The grey background is a little ugly because it's square and the "die" is round --- i'm not 100% sure, but i think if someone makes me a circle with a light grey background (i used #eeeeee here), i can insert that pretty easily.

Anyway:
* Fixes #430
* I reproduced the heisenbug #441, and then refactored the Api module to live in an anonymous function, so that may have fixed that (i'll probably close it optimistically)
* Might fix #405, since the greying of the dice will make a visual difference in any case in which it's not your turn