WIP on courses fix

[wip-abramson]

Description

Initial experimentation around fixing the courses application, so that a learner can take any lesson at any part of the course WITHOUT having to have completed all previous lessons up to that course.

Currently have links to any lesson working. However, more work is required to update the tracking and UI of completed lessons. No lesson link should be greyed out, but you should still be able to tell if a lesson is completed. There will also need to be some rethink of how we keep track of the place a learner is at in the course so they can return to it.