Marking lessons as cancelled

Problem/Motivation

Lessons that are cancelled could simply be removed from the lesson list, but that would make it difficult for students to note the exception from the regular schedule. It would be useful to have cancelled lessons marked explicitly.

Proposed resolution

* Add a field to lessons to mark their status, such as normal/cancelled. (More states could be added later on – maybe the "content fixed" thing should go here, see #1910870: Allow lessons to never be pushed/shifted.)
* Use the state to filter out cancelled lessons from ordinary week lists.
* Add a list of cancelled lessons at the top of week lists.
* Give cancelled lessons another appearance in the calendar view.
* Add another button to the lesson admin page, for cancelling lessons (rather than deleting them).

It should be considred to have a completely different content type for cancelled lessons – some kind of place holder which should never be included in summaries and the like.

Note: This is kind of a big issue, which will affect the structure and functionality of lessons. Until now, they are more or less shell containers for sections (with dates), but now they will become content of their own. This issue should most likely be coordinated with #1893044: Make lessons Field collections, not nodes..

Remaining tasks

Evaluate idea.
Implement.
Evaluate implementation.
Iterate.

User interface changes

(as described above)

API changes

(as described above)