Course nav - additional caching optimization

A follow-up to 📌 [PP-1] Convert course navigation block to SDC Active

Problem/Motivation

Currently we mark the current activity on the server side. This requires nav block cache invalidation of not only the current lesson but also the lesson that the student navigated from when going back to previous lessons. The current implementation is even less optimal as it invalidates cache of every lesson element when parent course status changes while only one or 2 lesson elements actually change.

Proposed resolution

1. Implement determining the current activity and disabling link in JS basing on the current path
2. Remove course status cache dependency from lesson nav element
3. Add a custom tag that will be invalidated when anything changes in lesson status except the current activity or when a new lesson status is created for a lesson and use that as the only cache invalidation tag for a lesson course nav element.