I still think this is a wrong fix.

All external calls to Compose.append with a string want to append that string literally. See for example https://github.com/mozilla-b2g/gaia/blob/master/apps/sms/js/message_manager.js#L185

Note that I don't see any "internal" calls to `append` now or to `insert`, I may miss something, but maybe the sanitization done in `compose.insert` could be changed to a simple assign to `textcontent` ?

What does call `insert` currently ?
* prepend => is not called anywhere afaik
* attachment replacing => always uses an Attachment
* append

`append` is called:
* in activity_handler: uses an Attachment
* in compose.js: as a result of an attachment picking, uses an Attachment
* in message_manager.js: one call with an Attachment, one call with a string coming from the SMIL, as a forward
* in thread_ui.js in this place

Now, `setMessageBody` is called:
* in message_manager when forwarding a SMS (our bug)
* when handling an activity with a body (pure text)

I think all this leads to removing this old sanitization in `composer.insert`.

Also, in the future, I think `setMessageBody` should also handle SMIL instead of doing this in MessageManager, but that's probably for another patch.

What do you think?