Fix an issue where transactions in mail were always rendered as text
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.
Reviewed By: amckinley
Maniphest Tasks: T12921
Differential Revision: https://secure.phabricator.com/D19968