Removes the tightly coupled upgrade/upsell ("notifications") panel from the Learning MFE core and replaces it with a pluggable widget registry system. The right sidebar now supports dynamically-registered external widgets, making it easy to add, remove, or replace sidebar panels without forking the MFE.
* feat: add functionality to see unit draft preview
* feat: add tests for course link redirects
* fix: course redirect unit to sequnce unit redirect
* fix: test coverage
