R1:ea131bb2efb9

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: {F432383}

Reviewers: joshuaspence, fabe

Reviewed By: fabe

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D13006