[INTERNAL] moved FollowingActivitiesManager into session scope

Also cleaned up this class.

Change-Id: Iee076217ec1740c5c1261c117ce9b557eda825e8
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1589
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>