Enforce max folder depth as a database constraint

[brianhelba]

This pushes max folder depth enforcement to two extremes:

• In the model validation layer, where it's enforced via full_clean (or Form validations)
• In the database, where it's enforced as a fallback, which should typically never happen

This also starts the expectation that at Folder instantiation, the choice of parent will be decided (either None or specified). Changing a Folder's parent after instantiation confers a responsibility to update all associated denormalized fields (e.g. depth).

[brianhelba]

I realized refactoring this was necessary as part of creating a Folder factory. It should be simpler to review as a separate change. I also think it's an improvement in its own right, as we now have fewer things happening via signals (which are hard to maintain) and more database constraints.