Bug 1080820 - [Messages][RTL] Sticky header does not take the whole width

- Adjust sticky header position and force background color in white instead of transparent.
problem with this background-color is that we don't see the header while the app is loading (it's showing the first time we call refresh, or when we scroll). Before the background-color we saw the element behind.

So there are different ways to fix this:
* call StickyHeader.refresh() when we append the first thread
* add `background-color: white` inside the sticky header libreary, when we set the moz element
* set the moz element for the first thread when we append the first thread. We can possibly use https://developer.mozilla.org/en-US/docs/Web/API/document.mozSetImageElement when we create the first header. (and then we need a dummy -moz-element in the CSS); that will get overwritten when StickyHeader.refresh is called.

* 1st solution is easiest
* 2nd solution could make it work better in all apps that use the lib
* 3rd solution could make the header appear earlier