Implement Login in API

[rieglerthomas]

• [x] Validate Token
• [x] Safe ownership in database when uploading a game
• [ ] ~Check ownerhsip when updating a game~ (Will be removed, see #41 )
• [x] Check ownerhsip when deleting a game
• [x] Check ownership when reading a game by id
• [x] read all games return games of the user who is logged in
• [x] Update tests
• [x] Remove not used methods