Begin modularizing Herald field values

Summary:
Ref T8726. I'm primarily trying to modularize tokenizer values so we don't have to update JS to add a new one.

This is ultimately the blocker for "select" custom fields working in Herald.

This inches us toward that. I'm not modularizing conditions or control types in this round, but hope to end up with hard-coded conditions (which are highly general and very rarely change), hard-coded control types (which are also highly general and very rarely change) and completely modular fields and values (which have mid-to-low generality and change frequently).

Test Plan: Used UI to interact with "none", "text", and new-style "select" controls. No actual support for tokenizers yet.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8726

Differential Revision: https://secure.phabricator.com/D13613