Multiple game owners

[oestrich]

Switch games from belongs_to :user to has_many :admins

Create a new join table that let's multiple users administrate a game. No special roles at the moment, all admins can manage the full aspect of a game.

Maybe we keep the current belongs_to :user and rename to be the "owner" so we can tell who originally made it and maybe who should have slight extra power in the future. Other systems have the concept of owner and admin, and just admin.

Criteria:

• [ ] Create table for the users <-> admin <-> game join
• [ ] Page to manage game admins
• [ ] Update any authentication check to check for being a game admin instead of just the single owner