Split Detail Endpoint includes Variant Detail lazily-initialized display_name

This resolves a problem with the test_track_rails_client where on the server we were leaning on lazily initialized records to display certain pieces of information. The client would only display variant_details that had been updated, so unless the data was updated manually the rows would be empty. This allows a default value of the Split variant name to be displayed if the administrator display_name has not been set.