Bug 1064502 - [RTL] Contacts List - Letters should not overlap contact names and faces. r=sergi

Nit: Maybe just for explicitness we could specify that this is for the `html` element. Like

```css
html[dir="rtl"] #view-contacts-list [data-type="list"] {
  ...
}
```

Just because we use it below with the `not` clause.