Add the ability to add games to user library

[hegedus-mark]

This one is a bit tricky. We'll have to think about how to do this. here's the initial idea:

Either add the game library to the IdentityUser or create a new AppUser entity and connect it with IdentityUser

Create a Post endpoint that adds the game to our DB if it's not there yet (if it came from an api query it might not be in our database yet), and add the reference to the user's game library

Add a remove game from user library endpoint to pair this

[hegedus-mark]

We'll also have to make sure, the user can only add games to his own libarry, which means we have to read the userID from the cookie and compare it with the request's params