Link games to an opponent

[danbee]

• [x] Game creation should allow me to pick an opponent
• [x] Show who I'm playing in the games list
• [x] Games list should show games for which I'm an opponent
• [x] Game should show who I'm playing against
• [x] User should not be able to see a game if they're not the user or opponent
• [x] User should not be able to update a game via the API if they are not the user or opponent