Support "Select" custom fields in Herald rules
Summary: Fixes T5016. Ref T655. Ref T8434. Ref T8726. For "select" custom fields, permit construction of Herald rules.
Test Plan: {F602997}
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T655, T5016, T8434, T8726
Differential Revision: https://secure.phabricator.com/D13618