[FIX] #838 Follow mode states are not cleared when users leave the
session 

Added missing logic to handle user left events.

Change-Id: Ie5d18be29aa42fe2a243c60ae2b14ef1ce036642
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1979
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>