When replying or fowarding an HTML message, we now send both plain text and
HTML bodies as a multi-part mime message. We take special care to ensure the
message bodies are in their own multi-part block and do not interfere with
any additional attachments to the message.
bug 3060920
Backport-Of: I2fc3cb4e1f65bcc28486a62731b44b0ee0a99719
Change-Id: I89ec2795b55ceb7472a8ee3db2dc8f50cf537d9c