[UI] display the session nickname in session chat

To be more consistent with the rest of the UI the session chat now
displays the session nicknames instead of the XMPP nicknames.

The behavior for 1-1 chats is not changed by this patch and will still
display the XMPP nicknames inside those chats.

Change-Id: Ia2577405c1c7626bd3de6369e87ee03cbe0e9f21
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1859
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>