FEATURE: Enable loading slider by default (#22439)

Followup to d51baa3bb3b08c5a7df6c22593d0162be6543147

Also includes: Force full rerender of post-stream widget when switching topics. This ensures that plugin/theme decorators are re-run when we switch between topics with the loading slider enabled.

app/assets/javascripts/discourse/app/components/scrolling-post-stream.js

@@ -372,7 +372,7 @@ export default MountWidget.extend({
   },
 
   didUpdateAttrs() {
-    this._refresh();
+    this._refresh({ force: true });
   },
 
   _handleWidgetButtonHoverState(event) {

config/site_settings.yml

@@ -396,7 +396,7 @@ basic:
   page_loading_indicator:
     client: true
     type: enum
-    default: "spinner"
+    default: "slider"
     choices:
       - spinner
       - slider