feat: typed PK identifiers for Units/Subsections/Sections

[bradenmacdonald]

This is an optional extension to #534 which further breaks Container.PK down into Unit.PK, Subsection.PK, Section.PK key types.

For now it works smoothly and has a very small diff to implement. But I am worried that it could add more complexity (and require more casting) than it's worth down the road? Hard to say. Since we don't have much difference between the container types at the moment, I'm not sure what the trade-offs are here.

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[kdmccormick]

Looks good to me 👍🏻

(If we end up finding the casting too cumbersome, then I think that's an indication that the container subclass models shouldn't exist.)