Publish rating changes to RabbitMQ for LeagueService to use

[cleborys]

Right now only game results are published to RabbitMQ, but since the league service doesn't want to duplicate the rater logic, what it really wants is an event published on rating change.
The expected format of these messages is e.g. available in these tests.
When this is done the routing keys in the league service should be updated accordingly.

(Right now the league service actually listens for game results using the "success.gameResult.create" routing key, but it then tries to parse the message as if it was an event as described above and hence fails.)