[IMP] mail: parsing emails with several html parts

If an email contains several text/html parts inside a multipart email, the previous code was only keeping the last content part.
The Content-Type: multipart/mixed allows several independent part (RFC1341 7.2.2), so two html is technically valid.
With this patch, the two parts are concatenated. (opw 614755)

Modify append_content_to_html regex to make sure the regex keeps the content of the html instead of removing it.
e.g.: "123 <html> 456 </html> 789" used to be stripped to "123  789" while we expect "123 456 789"