Call _post_downgrade_### after downgrade migration is run
    
    Fixes bug 1195470
    
    Tests are currently only run after a upgrade during the database migration
    unit tests and never during a downgrade. This change calls an optional
    _post_downgrade_### method after running the downgrade migration to allow
    it to be tested as well.
    
    Change-Id: I592424b464b91c2be9e253402986e673ce33c8d0