Bug 1091486 - [SMS] Switch to MMS when entering a email recipient thread

Dividing this may introduce different issues, eg. it regressed "duplicated draft" issue again. See https://bugzilla.mozilla.org/show_bug.cgi?id=1079824 for more details.

Well, to be honest, taking into account all that risk and complexity of ThreadUI & Composer interaction, I'd rather go simple way in this bug, eg. use ```Threads.active.participants``` and ```Utils.isEmailAddress(this.number)``` inside ```hasEmailRecipient``` (I admit that it's hard dependency to ```Threads```, but it seems reasonable until we unify recipients and participants to have only one collection) + some code that prevents "type conversion" banner from appearing in email thread, eg. check for "email" participants inside ```ThreadUI.onMessageTypeChange```.

With this we'll just have two relatively small changes inside Compose and ThreadUI with minimal risk to break something :)

What do you think?