Refactor states

The idea is to keep state management in a single place: the backend.

 1. Server returns [state](https://github.com/eladb/frienderers-backend/blob/278f120bb6c7cc6c5e0715a68d9a2bcc5b4b931d/game.rb#L78) field in every response.
 2. `TKServer` updates their `state` property whenever we send any request to the server.
 3. `TKAppViewController` observes `[TKServer sharedInstance].state`
 4. Whenever the state _changes_, `TKAppViewController` folds back the navigation 
    stack and pushes on the view controller scene based on the new state.