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.
> 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.

Definitely! Probably it will be easier to do in the scope of separating participants\report panel from thread panel, I'm really eager to do this during some future sprint in background, you know I love refactoring stuff :)