Offline functionality

[Hachitus]

We should implement a possibility to play the game even when there is no internet connection. How we do this and what is part of the engine and what is handled outside of it, has to be thought.

We can use straight up service worker in this, as it should have enough support for us. We can basically store the data locally, e.g. to localstorage or somewhere else, if the http request to server does not succeed.