Bug 1052442 - [sms] We should never close the activity when viewing the message report. r=julien

Just showing ordinary back button for participants panel and _other_ close button for the report panel + hide _activity_ close button if in activity mode.
specifity war !
tadadadaaaa daaaa tadadadaaaa daaaa (insert star wars music here)

A proper fix would be https://bugzilla.mozilla.org/show_bug.cgi?id=961572 but I'm fine with this solution given it's a 2.0+ bug.