List all lessons in the input API

[encukou]

Currently, naucse loads lesson content on demand when a lesson is included or linked from a course. To get a list of all sessions in a course, all sessions need to be rendered. This complicates the logic (and caching).
The course API should get an extra_lessons key with lessons not appearing in any materials. For older API versions, this can be configured in link.yml files.

[encukou]

This is now half-done: extra_lessons is required for compiled courses, but the error message comes too late -- when the course is used, rather than when it's compiled.