Allow administrators to configure "Email Format" settings for lists

Summary: Ref T8387. Mostly completeness, but you might want to choose html vs text mail.

Test Plan: {F468203}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8387

Differential Revision: https://secure.phabricator.com/D13137