- Email generated from mass mailing will have its own layout with internal css.
- strip_classes is removed from mail_message because mail client are now supporting
  internal stylesheet. we need it for make mail responsive.

mail: manually removed classes from incoming mail body

Because we removed strip_classes from mail message body field because we want to
allow classes in outgoing mail to make it responsive but classes are not removed
from incoming mails so here we manually removed classes from incoming mail.

opw-1836556