Fix issue where deleting a Cluster or Host tries to delete all policy_events

https://bugzilla.redhat.com/show_bug.cgi?id=1167110

:dependent => :nullify has been removed in favor of adding a purger for policy_events table.  To try to start generalizing all of our purgers I created a PurgerMixin that at the moment focuses on "by date".  See the second commit for details.

@blomquisg @gmcculloug Please review.
cc @chessbyte 
No, that's exactly what I just undid.  The code never walks the reverse relationship (in fact it's not even defined in the model), so it's a "safe" trade-off.