Bug 1053708 - Make SMS main User Interface RTL-friendly

Deep in my mind I had an idea that we can adopt css-variables for this RTL-override use case. Not sure if it's good or feasible and what approach to use, but I was imaging something like this (this may increase chances that developer won't forget to update RTL version):
```
:root (or ".writing-direction-aware-variables") {
  --edit-label-translate-x: 40px;
  --thread-icon-backgroun-position: left center;
}

html[dir="rtl"] {
  --edit-label-translate-x: -40px;
  --thread-icon-backgroun-position: right center;    
}

.edit-label {
	transformX: (var(--edit-label-translate-x));
}

.thread-icon {
	background-position: var(--thread-icon-backgroun-position);
}
```