Play sounds when placing pieces on the board

[kreibaum]

This can't be done in pure elm, you'll need to use a port and play the sound in javascript. We also need to decide what kind of sounds we want and find (or produce) free sound assets for this.

What kind of sounds do we need?

• [x] Placing a piece or union on an empty square
• [ ] Forming a union
• [ ] Taking over a union
• [ ] Promoting a piece
• [ ] Game over sound
• [ ] Restart move

[kreibaum]

Currently sounds only play when you place it, not when you get the info from a websocket. That isn't good.

[kreibaum]

I would love for someone to pick appropriate open sounds for all these cases. That's why I added the help wanted flag. You don't have to actually implement this ;-)