Referential integrity for class information in report data

Due to time constraints and the lack of the existence of historical catalogs in PuppetDB during the run-up to PE3.1 release, we added some fields 'containment_path' and 'containing_class' to the events table for reports.

These are currently populated with simple string values that are received from Puppet. In an ideal world, this data is really part of the catalogs rather than part of the reports, so there should be some way to join the tables together and only store that data in one spot. Basically, we have no referential integrity for this data at the moment, even though it should absolutely be possible for us to have referential integrity once historical catalogs land.