Track how many columns use a particular trigger

Summary:
Ref T5474. In 99% of cases, a separate "archived/active" status for triggers probably doesn't make much sense: there's not much reason to ever disable/archive a trigger explcitly, and the archival rule is really just "is this trigger used by anything?".

(The one reason I can think of to disable a trigger manually is because you want to put something in a column and skip trigger rules, but you can already do this from the task detail page anyway, and disabling the trigger globally is a bad way to accomplish this if it's in use by other columns.)

Instead of adding a separate "status", just track how many columns a trigger is used by and consider it "inactive" if it is not used by any active columns.

Test Plan: This is slightly hard to test exhaustively since you can't share a trigger across multiple columns right now, but: rebuild indexes, poked around the trigger list and trigger details, added/removed triggers.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T5474

Differential Revision: https://secure.phabricator.com/D20308