Game board images always load current game state.

[cjsaylor]

While this is nice normally, if you reload slack or visit it from another device, it looks like all the images are the same (current state).

Need to include a PGN path as part of the parameters so we can still load the game from the backend and progress to the last part of the provided PGN.

[cjsaylor]

Implemented, but uses the FEN and a signature to keep non-official game linkage.