Make lessons mixable and matchable

[reid-a]

Feedback from the SC21 BoF indicated that organizers should be able to mix and match lessons to build up workshop. Review the lessons and ensure this is true, and that manual sorting works for each lesson.
See the BoF Codi for additional details, and also the BoF results data for details.

[reid-a]

This is closely related to issue #97 -- one way that multiple durations can be achieved is by stringing together several lessons. The different durations would then also correspond to different coverage of material, rather than the alternative of having the durations reflect the same material being covered at different speeds.