R1:252b6f226056

Provide basic scaffolding for workboard column triggers

Summary:
Depends on D20278. Ref T5474. This change creates some new empty objects that do nothing, and some new views for looking at those objects. There's no actual useful behavior yet.

The "Edit" controller is custom instead of being driven by "EditEngine" because I expect it to be a Herald-style "add new rules" UI, and EditEngine isn't a clean match for those today (although maybe I'll try to move it over).

The general idea here is…