thread: Fixup link URLs for emails

This patch adds an 'email' format to ThreadBody::display() which allows
formatting the html/text bodies specifically for email. Previously, there
was ambiguity between the html view (which included inline image support and
clickable links) and the email view, which requires neither.
Merge pull request #929 from greezybacon/issue/926

thread: Fixup link URLs for emails

Reviewed-By: Peter Rotich <peter@osticket.com>