[FIX] session overview was wrongly sorted

The session overview display nicknames but the viewer was sorted with
the real JID addresses.

Change-Id: I3122d96b758355ed2f90106a4ac9616122f20f42
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1609
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>