Respect "metamta.email-body-limit" when building mail HTML bodies

Summary:
Ref T13151. See T11767. See PHI686. Although we limit outbound mail text bodies, the limit doesn't currently apply to attachments, HTML bodies, or headers. T11767 discusses improving this in the general case.

In the wild, an install hit an issue (see PHI686) where edits to Phriction pages generate very large HTML bodies. Check and respect the limit when building HTML bodies.

If we don't have enough room for the HTML body, we just drop it. We have the text body to fall back to, and HTML is difficult to truncate safely.

Test Plan: Added unit tests and made them pass.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13151

Differential Revision: https://secure.phabricator.com/D19489