Fix an issue where transactions in mail were always rendered as text

Summary:
Fixes T12921. Currently, we call getTitleForHTMLMail(), but that calls getTitleForMail() which forces us into text rendering mode.

Instead, have getTitleForHTML/TextMail() force the rendering mode, then call getTitleForMail() with the desired rendering mode.

This causes stories like "epriestely added dependent tasks: x, y." to appear as links in email instead of plain text.

Test Plan: Used bin/mail show-outbound --id ... --dump-html > out.html to verify HTML mail.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12921

Differential Revision: https://secure.phabricator.com/D19968