Actually save FPRule after modify counts
Actually save FPRule after modify counts

refs #6611

Use Django's F() construct to update the counts in a way that avoids race
conditions from several instances of the same rule running at once.

michal-ruzicka/archivematica,artefactual/archivematica,eckardm/archivematica,artefactual/archivematica,sevein/archivematica,sevein/archivematica,eckardm/archivematica,eckardm/archivematica,michal-ruzicka/archivematica,eckardm/archivematica,artefactual/archivematica,artefactual/archivematica,sevein/archivematica,michal-ruzicka/archivematica,michal-ruzicka/archivematica,sevein/archivematica