Display review statistics on `event#show`

* Query event statistics in `EventsController#show`
* Optimize N+1 queries in `EventsController#show` with `includes`
* Add `reviewing_users` relationship to `Event` to optimize queries
* Extract partial for event statistics and update `event` partial
* Render statistics partial on `events#show`

https://trello.com/c/thwjjzVD