Centralize rendering of application mail bodies

Summary: This is a minor quality-of-life improvement to prevent D2968 from being as nasty as it is.

Test Plan: Ran unit tests; generated Differential, Maniphest and Diffusion emails and verified the bodies looked sensible.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T931

Differential Revision: https://secure.phabricator.com/D2986