Folder trees

[rebeccacremona]

Django MPTT is now unmaintained https://github.com/django-mptt/django-mptt

Let's figure out our exit strategy.

[rebeccacremona]

Maybe we can re-use H2O's content tree logic.

[rebeccacremona]

If/when we end up having to do this: we are aware that some of the trees have become corrupted, probably due to two users making changes at once. We haven't yet investigated how many, or how to fix.