Allow triggers to be attached to and removed from workboard columns

Summary:
Depends on D20286. Ref T5474. Attaches triggers to columns and makes "Remove Trigger" work.

(There's no "pick an existing named trigger from a list" UI yet, but I plan to add that at some point.)

Test Plan: Attached and removed triggers, saw column UI update appropriately.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T5474

Differential Revision: https://secure.phabricator.com/D20287