using the shop during preparation phase

[bzns]

• Update the game and user instances based on the request received from the client
• Return the updated stats and gold balance at the next /games/{gameId} update
• Return an error message if client tries to access the shop during battle phase or does not have enough gold to make the requested purchase

Estimated time: 7h
This task is part of user story #15

[Thahit]

650640d