Fix crash when applying deletion rules

When deleting a document references by other, if that refence is
defined on an abstract document, the operation fails, because it tries
to apply deletion on the abstract class which doesn't have a QuerySet.

Fix is simply to ignore document classes which are defined abstract when
applying rules on all classes referencing the document.
Merge pull request #825 from idlead/fix/reverse_delete_rules_on_abstract_documents

Fix crash when applying deletion rules