Views show "on behalf of" when principal differs

Adds notice methods around principal. Unfortunately, "entities that have
principaled" doesn't make sense, so I went with "principals". That lead
to changing all of the methods to be consistently named as the plural of
the role they represent.
Add principal to fake notice seeds
Map entities from legacy data correctly

* Sender_LawFirm becomes Sender (name and address)
* Sender_Principal becomes Principal (name only)
* Sender_Attorney becomes Attorney (new role, name only)

Also refactored a bit to make adding/changing entity mappings easier.