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.
> on a side note, it's quite "funny" that the "close" button for the report view is really the "back" button node. Yay.

For me it's still not clear why report panel has close button while participants has back button, I know there is a reason for that, but I still don't feel it :)