Users can technically create games for platforms defined by other user

[KowalskiPiotr98]

Something that I couldn't be bothered to fix in #19: users can manually get ids of platforms from other users and set them for their own games.

This doesn't really do anything, as permission checks mean neither user will really be able to see anything that the other user has done, but the database state might get funky when doing so (things like counts and such might be different to the actual lists, depending on the implementation that's also not here yet).

To solve this, just make a check somewhere to ensure the user is authorised for the platform when setting it for the game.