Cache game state for Unlimited games

[ssoelvsten]

Since the unlimited game mode is not timed, there's no reason to not allow people to play it even more slowly over multiple sessions.

• Make the cache id also include the index for the game. This allows us to refetch it later. We still need the longest word to catch the dictionary having been updated.
• Extend the clear function to take an (optional) argument of the accumulated score.
• When fetching a new game in Session, call clear on the cache with the accumulated score.

The following might also be worth considering

• Add a pop-up asking the player whether they'd like to continue their previous game or start a new one.