[REFACTOR] Move invitation.hooks package to core

Change-Id: I744b88e3bdc49eb3ef73b8843db2d71bf2c702fd
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1490
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Reviewed-by: Bernd Bieber <brotlee@zedat.fu-berlin.de>
Reviewed-by: Andrej Szaffranietz <a.szaffranietz@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>