[HPT-264] Write module that let something function as a level in a hierarchy: handles ordering somewhow, parent/child sets

Handles ordering somewhow, parent/child sets.

This wound up touching a lot of code, often fairly lightly.  One change to note well is that Page and Paged methods that formerly accepted/returned models (parent=/parent, for example) now accept/return PIDs.

Under the hood:  Page and Paged are now derived from a superclass Node, which handles both sibling and descent relationships.  More models can be derived from Node easily and decorated with any fields and behaviors that they require.  So, for example, we can invent Section between Paged and Page, or Volume above Paged (for a periodical, perhaps) and have its hierarchial characteristics already catered for.