Featured Games API Endpoint

See: https://github.com/cvan/galaxy-api/issues/41 https://github.com/cvan/galaxy-api/issues/40

The GET endpoint will return an array of game slugs of the featured games.

I've opted to use PUT for the edit of featured game. PUT will modify the genres pages in which the game will be featured in. (Edit is basically DELETE + POST)

r? @cvan @mattbasta @CharlesChong