This commit extracts the storage part of the route-scroll-manager into a dedicated service. This provides a key/value store which will reset for each navigation, and restore previous values when the user uses the back/forward buttons in their browser.
This gives us a reliable replacement for the old `DiscourseRoute.isPoppedState` function, which would not work under all situations.
Previously reverted in e6370decfd. This version has been significantly refactored, and includes an additional system spec for the issue we identified.
ed1dece517