Improves sortable ux

- Fixes multi-select functionality.
- Updated other books to be sticky.
- Added some general intro/desc text.
- Updated sort boxes to be collapsible.
- Cleaned up other books styling.

resources/sass/_layout.scss

@@ -268,6 +268,11 @@ body.flexbox {
   }
 }
 
+.sticky-top-m {
+  position: sticky;
+  top: $-m;
+}
+
 /**
  * Visibility
  */