Add support for changing configuration while playing 

When the user clicks the cog button on the play page, they are taken back to the
new page, and then they have two buttons, "continue game", or "new game". If
they click continue, they are taken back to the play page, to where they can
continue from where they left off.

### To Do:
  - Detect when settings change in such a way that the "continue" button no
    longer makes sense (Changing any variant options)
  - Add support for game specific config settings. Basically add a "what do you
    want to save" and "restore saved stuff" callback for the game to
    (optionally) implement.