Implement LocalStorage

[ovidiubute]

• Game progress should be saved to LocalStorage
• This will allow users to continue the game even if they close their browser/tab
• What needs to be saved?
  • Some sort of user id, we don't want a full login system yet so we'll have to generate a UUID for him and save it
  • It's 2017, we don't care about browsers not supporting LocalStorage, so no cookie workarounds 😄
  • The game needs to be paused when leaving the app and the state must be persisted
    • This is required because otherwise you will reload the game and the timer will start immediately and it will be confusing
  • Every move a user makes needs to be saved
    • This will be easier in the long term than just saving a snapshot of the game, we can implement replays for example or even (wait for it) have some sort of async multiplayer mode
    • This needs to be light so a format like [ [0,'r'], [1, 'l'], [3, 'u'], [2, 'd'] ] should be used (tile index 0 to 14 and direction 'r' = 'right', etc...
• On new game we can clear the history until we have a backend to save them (way long term)
• We'll need full 100% unit-test coverage for this functionality because it will be very easy to break stuff and it's very hard to debug local storage issues
• Also need to take care of users having inconsistent data in local storage due to application bugs
• There are literally dozens of LocalStorage libs out there, need to research on which one is the lightest and suits our needs

[ovidiubute]

Update after 6 years (how time flies):

• Hitting reload will make you lose your progress, this sucks. Have a confirmation dialog (browser dialog is good enough).
• No need to use a LocalStorage lib, just use the API directly
• Hitting reload should check if there is a game currently in progress, and if yes, load it, but start in pause mode so the timer doesn't just keep going
• We don't need a user id, a lot has changed in 6 years, people hate tracking nowadays 🥲