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