Split listings table into Properties and Listings

[gabeklavans]

Properties table will have most of what's in the current listings table, but remove refs col.

Listings will contain the link to the listing and a foreign key to the Properties table.

When the UI requests all the listings, it'll get them back in the same JSON structure as now, but with refs as an array of strings. This can either be done with a join of some sort or just in Go by the server.

This will enable better control of removals and notifications.