Unchanged behaviour rework

[Bluerberry]

Remove the unchanged field from programs, courses, graphs, and from all database entities. From now on, unchanged controllers like links, domains, subjects, relations and lectures get purged when leaving the page.

[Bluerberry]

This involves no longer keeping track of unsaved or unchanged, and adding a prune method to Graph that removes all empty nodes/relations/lectures. Prune should be triggered on editor navigation (?)