Add dashboard for events with top critic reviews + refactor

**Add dashboard for events with top critic reviews:**
* Add route, controller, and view for top media events dashboard
* Add method to `Event` for events with highest media review scores
* Add link to application layout

**Refactor: Add counter cache for user and media review counts:**
* Create migration to add columns to `events`, setting current value from data
* Refactor `with_review_statistics` method to remove unnecessary `COUNT`

**Refactor: Use DB 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`
* Add `default_scope` to `Event` to always eager load statistics
* Remove `with_review_statistics` method from `Event` (and controllers)
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