Implemented a disabled state for Game Board Cells.

When a User adds a piece to the board, that cell (or box) is no longer
available to add new pieces. I have labeled this a 'disabled state'.

During a disabled state, the game board cell will show a red background
if the User hovers over it. I may decide later to remove the 'pointer'
cursor in this state as well.
Enhanced UX for selectable pieces.

Their background colors are now a shade of green which indicated "Go"
better than Blue - especially in contrast to the Red of the disabled
state.

The mouse cursor is shown as a 'pointer' (or Hand) to indicate
selectability.