Mostly make the editor UI for triggers work

Summary:
Ref T5474. This provides a Herald-like UI for editing workboard trigger rules.

This probably has some missing pieces and doesn't actually save anything to the database yet, but the basics at least roughly work.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T5474

Differential Revision: https://secure.phabricator.com/D20301