KAZOO-3217: conferences: play name on join feature

I believe `wh_media_util:media_path/2` expects the AccountId versus the Db for creating the path to play. It may not matter since `wh_media_file` resolves this to the AccountDb, but it is fewer characters to transmit, easier to read when trying to see why a prompt failed to play, etc. For the sake of consistency, let's use AccountId instead.