Improve UX for Diffusion mail fields
Summary:
- Give the fields names and descriptions.
- When new, default-disabled fields are added, disable them by default even if there's already a config.
- Be a bit less hacky about $faux_spec.
Test Plan:
Reviewers: joshuaspence, fabe
Reviewed By: fabe
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D13006