In the past, our loading spinner implementation used Ember's loading substate. That meant that, when the site setting was toggled, there would be fundamental changes in the routing behavior.
This commit simplifies things so that the (non-default) loading spinner implementation is purely a styling thing, and behaves exactly the same as the spinner which appears under the 'slider' configuration when loading takes too long.
This does involve a slight UX change. Now, the entire page will be replaced by a loading spinner instead of just the relevant `{{outlet}}`. We strongly recommend sites use the new default 'slider' behavior.
This brings the functionality from https://github.com/discourse/discourse-loading-slider into Discourse core. Default behaviour remains the same - the new slider mode can be enabled using the new 'page_loading_indicator' site setting.
