Properly escape text/plain email bodies

If there characters in the plain text body of the email that appear like
HTML tags, for instance

From: <sip:527772432@172.18.0.2>;tag=952422a9dd1ap1a6o1

The <sip...> part would be removed by the Format::striptags() call in
Format::sanitize().
Assume text from the web user interface is not html
Merge pull request #652 from greezybacon/issue/htmlchars

Properly escape HTML chars in text/plain email bodies

Reviewed-By: Peter Rotich <peter@osticket.com>