Prevent moving items from a tree to another tree

[lvps]

We may have a Polito root, a SomeSchool root, a SomeOtherSchool root, and so on: make it impossible to move items across tree boundaries, so we don't accidentally move a RAM stick from a school pc to our lab if we type the wrong code.

But leave a way to override this check, since we need to move donated computers to SomeSchool: maybe place it in the bulk and uncommon operations page? See #25 and #28.

[fabmazz]

Maybe we can add a "donation" flag to the SomeSchoolRoot, requiring a confirmation from the administrator for a move or some kind of lock down procedure that can only be activated by an admin user.