Previously we were implementing scroll reset/memorization on a per-page basis. Many of these approaches relied on the `didInsertElement` hook, which is no longer appropriate since Discourse changed to use the 'loading slider' strategy for page transitions.
This commit rips out all of our custom scroll resetting/memorizing, and implements those things in a generic service. There are two features:
1. After every route transition, scroll to the top of the page
2. When using browser back/forward buttons, restore the last known scroll position for those routes
To opt-out of the behaviour, individual routes can add a scrollOnTransition boolean to their RouteInfo metadata using Ember's `buildRouteInfoMetadata` hook.
