Fully modularize Herald field values

Summary: Ref T8726. This gets rid of all the VALUE_* constants and lets Fields provide arbitrary typeaheads without upstream/JS changes.

Test Plan: Used all tokenizers. Used "Another Herald Rule". Grepped for all removed constants.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8726

Differential Revision: https://secure.phabricator.com/D13616