Subnet types

This includes two things:
1) replace the initial subnet types list with a fuller list, and related refactoring
2) the first iteration of the NetworkQuery API/helper class for grabbing interfaces, etc for hosts in the context of a particular subnet type.

1) is largely done here; 2) will need more work, but that could be done either as part of this PR, or as a follow-on PR after merging this one, since it's not referenced anywhere yet.
I'd add `required` named scope and use it here so at least part of this condition is evaluated in SQL