[REFACTOR] Move Activity classes to "activities" package

There is no parallel class hierarchy of Activities anymore, which were
structured in the "activities.business" and "activity.serializable"
packages.

Change-Id: I7ccf0ae678e8e45a5adbf0fa8d3f3113e9fa8ec0
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1527
Tested-by: Jenkins CI
Reviewed-by: Damla Durmaz <ddurmaz88@gmail.com>
Reviewed-by: Arndt Lasarzik <alasarzik@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>