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