Preserve active games for longer

[jbowens]

Presently incomplete games are removed from memory after 72 hours and from disk after 24 hours: https://github.com/jbowens/codenames/commit/76610894476baa75ce041f613136f5fe75c78ff2

The effect is that we only guarantee your uncompleted game will be around for 24, but if you're lucky and we don't redeploy it'll be around longer.

It'd nice to extend this to allow asynchronous game play over an extended period of time. We have an 'updated_at' field on games now, and that could be used to preserve games for longer. We probably would need to rework the kv keyspace, because right now games' keys are prefixed with their created at time to allow for efficient deletes of expired games.