Utilize a media's elemental_id when grouping it into a multi video

[bctcvai]

Multi video feeds contain 1-N media, currently only by listing the ids (primary key) of each child video.

If we add an option to support listing the media's elemental_id, then a multi can be cloned more easily from one project to another or one deployment to another. The elemental_id of the media should be the same for a given media across multiple deployments or clones on the same deployment.