Bug 1096330 - [Messages][RTL] Correctly handle recipient pills. r=julien

The Story: ```flex-grow: 1``` is set by ``` #messages-recipients-list > .recipient[contenteditable=true]``` to override BB styles, so I see two ways here:
* Current one, doesn't touch ```#messages-recipients-list > .recipient[contenteditable=true]``` to leave these BB overrides alone, but to override ```flex-grow``` we need ```#messages-recipients-list > ``` part to be specific enough.

* Divide ```#messages-recipients-list > .recipient[contenteditable=true]``` into ``````#messages-recipients-list > .recipient[contenteditable=true]``` (to have only BB overrides) and ```.recipient[contenteditable=true]``` to have the rest of styles, as the result we won't need ```#messages-recipients-list > ``` part to override ```flex-grow```.

The first one seemed simpler to me, so I chose it, but what option do you prefer?