[meta] Rethink structure for lessons

There are several issues concerning lessons that suggest that the information structure around lessons change.

Lessons are currently nodes, and are only (more or less) used as shell containers for references to a course section combined with a date/time.
Several things points towards making lessons field collections, rather than nodes, and there are also things pointing towards extending the information stored/managed by lessons.

Related issues:
#1893044: Make lessons Field collections, not nodes.
#1911758: Marking lessons as cancelled
#1910870: Allow lessons to never be pushed/shifted
#1911732: Location for lessons
#1910922: Sums of remaining and total time, not only number of lessons
#1910506: RSS feed for course offering, with latest changes on top

Also: There's a suggested issue of being able to work with course outlines + section in minutes/hours, rather than the general "size". This would probably have significant implications for lessons.