Forum UI update tweaks Moodle 3.8

Following on from final reviews of the Forum UI updates for Moodle 3.8 we want to make the following changes:

1. Make the new Nested UI design a user preference, not a forum display dropdown option. So the user by default sees the existing Nested view, unless they in their preferences select to see the 'Experimental Nested View' (new design)
2. Experimental nested view in RTL does not show the groups icon; to be added.
3. The new UI design should only be applied for the nested view (as per 1 above). All other forum pages (discussion list page, new grading page, new reporting/export page) are to remain as closely as possible to the existing Boost theme.
4. The page header with course title, bread crumb and search option is to remain as per standard boost
5. Forum name to be displayed below that as per standard boost
6. In the new grading interface emphasize which user is being graded. Make graded username in the grading sidebar bigger, add icon, consider highlighting graded user posts (UX to review)(MDL-66959)
7. Review of 'Grade saved' message when no changes to grade or feedback have been made (MDL-66958)
8. Check accessibility for keyboard and screenreader in the grading overlay. Ensure it has the right focus and content underneath is not found. (MDL-66383)
9. Review option to grading user filter to easily get to ungraded users (MDL-66381)