Bug 1011604 - [Messages] Update to use <gaia-header>

Mmm looks like the last _"activity mode"_ case still doesn't work properly:
>When user in Thread panel, he can either go the Message Report panel (long tap on message) or Thread Participants panel (tap on header if it's group MMS). In Report panel we display close button that works just like back, in Participants panel we display normal back button.