Bug 1048717 - [Messages][Refresh] Display call button at the header of single recipient thread. r=schung

Even that I don't like using ```hide``` class, I found it less painful here when we have to show Call button only for single-participant non-email thread. Initially I was adding ```group-thread``` class and then managed Call button visibility through CSS, but it requires rule with ```!important``` + addtional class or js code for non-email check... Please share if you have better ideas! :) In future I think we make it easier when we have separate headers\panels.