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.

We need bug 961572 to resolve this technical debt :)