All Lessons now appear in the same section on the main page

All Lessons now appear in the same section on the main page

* Rationale for the decision: When restoring a course, Lessons that
get deselected (eg. because they're not lessons but topic replies,
exercises, etc) still used to show an empty section on the course
page. Moving them around takes A LOT of point and clicking. (If one
really doesn't like it this way, I believe it still takes only at most
as much point-and-clicking to create individual sections around each
Lesson).

* Also: now uses threads->sortedAll to have Lessons show in order on
course page