Fix to avoid the growth of the subject line in a multi-language environment. The Re: prefix is always added using the site wide locale to be able to trim it.
Broke out the check for valid mime types that ad_html_text_convert canconvert so a caller can check that it will work (the content repositoryis hard-wired to try to convert any text mime type when delivering itthrough an automagically-generated template created by content::init)