Implement bulk add_parent via ElementQuerySet.add_parent

A QuerySet-level equivalent of Element.add_parent, that adds a single parent to every element selected in the QuerySet. This could make CreateParentSelection faster and perhaps allow it to preserve ordering (#1469).

Having it operate on any QuerySet makes it very flexible, which can help when doing weird things in a shell, but it can also make it quite complex:

• Should elements that belong to a corpus other than the parent element be skipped or make the method crash?
• Should elements that would cause cycles be skipped or make the method crash?
• Should elements that already have the parent be skipped or make the method crash?
• Can we update existing paths even when the elements have wildly different existing paths, or should this only operate by adding then removing?

Some whiteboard work is required!