Bug 974867 - [MMS]Auto suggestion for email address r=Julien

https://bugzilla.mozilla.org/show_bug.cgi?id=974867
actually, it's not that easy to do and I'd rather do this in another patch.

We need to do it properly because we want to keep 'mms' if we remove an email recipient _but_ have an attachment or subject, and we want to keep 'mms' if we remove the last attachment but there is an email recipient.

So I think we need something more complex than what we have now to decide if the type should be MMS.

Therefore, let's do this in a future patch instead.