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

As Steve mentioned on the bug, seems that new header broke at least two features:
* In normal mode (not activity one) _back_ button in message report panel header should  look like _close_, but behaves itself as normal _back_ button.
* In activity mode it's more complex. Eg. user opens gallery and share image via messages:
 * Composer panel should have _close_ button that closes Messages and returns user to the Gallery;
 * If user sends message he is redirected to Thread panel (list of messages for some thread). User still sees _close_ button, that does the same as in Composer panel;
 * 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.