Switch to InheritanceManager throughout

This makes it easier to sub-class various models and select instances.  This is a requirement if you're starting from a tx_people based model and want to fetch the more specific version.

Current use case: tx_elections has a `FilerIdentifier`.  When I fetch the various `tec_filer_id` values associated with a person, I want the `FilerIdentifier`, not the `Identifier`.  To do that with this code, I do: `some_person.identifiers.select_subclasses().filter(scheme='tec_filer_id')`.