[PP-1] Convert course navigation block to SDC

Removing "PP-1" as 📌 Update legacy themes Active has landed, and adding as a child issue of 📌 Improve general appearance Active

WIP commit - includes a number of navigation improvements:

• adds single-directory components (SDC) for layout and styling.
• displays all but the current lesson as collapsed accordion lists, for compactness and better visual progress display.
• removes the link url from activities that are not available to visit, based on the course-level Free Navigation setting, or if the user is revisiting a completed course.
• improves the course navigation caching/placeholder logic, with appropriate cache lifetimes based on per-user and per-lesson display options.
• adds styling to each activity to indicate current / answered / unanswered activities.
• adds activity scores on course revisit.

Will do additional code cleanup and optimization, meanwhile any feedback on the looks and function is welcome.

Haven't reviewed the MR or functionality yet, but just changing status to make it clear there's something to look at here.

Added additional caching improvements and testing. Should be ready for review.