Add dashboard for events with top critic reveiws

* Add route, controller, and view for top media events dashboard
* Add method to `Event` to for events with highest media review scores
* Add link to application layout

https://trello.com/c/TdjUj0gg
Refactor event review statistics: review totals

**Add counter cache for user and media review counts:**
* Migration to add columns to `events`, setting current value from data
* Specify counter cache in user and media review models
* Refactor `with_review_statistics` method to remove unnecessary `COUNT`
Refactor event review statistics: review averages

**Refactor: Use database view to compute review statistics:**
* Add migration to create view with user and media review averages
* Create `ReviewStatisticsSummary` AR model for view
* Add relationship to `Event` for `ReviewStatisticsSummary`
* Delegate average methods from `Event` to `ReviewStatisticsSummary`
* Move score methods from `Event` to `ReviewStatisticsSummary` and delegate
* Add `default_scope` to `Event` to always eager load statistics
* Remove `with_review_statistics` method from `Event` (and controllers)
* Rename `with_review_statistics_by_average_media_review` in `Event`