[BDGR-145] Tree-based show structure

[markspolakovs]

Scrap the semi-arbitrary distinction between rundown and continuity items, instead have a tree-based structure - items either have media, are "rundown items". or are containers for other items (what we'd call rundowns today).

Still some logistical hurdles.

_{From SyncLinear.com | BDGR-145}