Write workboard trigger rules to the database

Summary: Ref T5474. Read and write trigger rules so users can actually edit them.

Test Plan: Added, modified, and removed trigger rules. Saved changes, used "Show Details" to review edits.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T5474

Differential Revision: https://secure.phabricator.com/D20302