Create a highscore endpoint

The endpoint should support the following.

GET /highscores

200: returns a JSON-object of top 10 users

204: no content in the collection

GET /highscores/{username}

200: returns a JSON-object with username and highscore.

204: user does not have a highscore

404: user not found

POST /highscores/{username}

200: return a string that says score is updated.

(no 404 since user will be created if not exist?)