Adds API endpoints and documentation for operator shelves.

r? @ngokevin
In the operator dashboard, operators—just as Scott can other types of feed items—are able to create multiple shelves and choose one to be "active". So there may be multiple FeedShelf objects with the same carrier/region pairs, but there can only be FeedItem with a carrier/region pair.

Does that make sense? Happy to listen to alternate ways to express/implement this.