Bug 1003843 - Follow up to Bug 951676 - [Messages][Refresh] Add specific image in case of more than one recipient in the list threads. r=schung

I believe if we adopt BEM-like approach it will become more like ```.threadlist-item-picture--default```.

```.threadlist-item``` is used to beat BB rules that are more specific (```span[data-type=img])```)

Currently I use separate class ```group-picture``` for the group thread case. Mainly because of logical separation (though I think it's not quite important) and method of text centering via flex (I like it :) but it probably can affect performance, so I was eager to apply it on group thread list items only). 

What do you think?