fix against issue https://github.com/globaleaks/GlobaLeaks/issues/415
added migration script for issue/415
fixed issue https://github.com/globaleaks/GlobaLeaks/issues/467
addressed issue https://github.com/globaleaks/GlobaLeaks/issues/482
issue/415 complexity reductions (thanks to vecna suggestions)
Fix bug in importing of models
addressed issue https://github.com/globaleaks/GlobaLeaks/issues/485
aligned migration script to last commit changes
fixed updating script against 4 populated db: 0,1,2,3->4