Add basic DNS record list/create/delete endpoints per zone
Remove server-side /zones/ endpoint

For now at least, I'm doing a per-backend /backends/{backend}/zones instead;
it was easier to build that way, and will probably be better for performance too.
Wire up basic list of all zones, across all backends, in the user interface:

 * Remove global ZonesController
 * Add backend.zones (a ZonesController) on the Backend model, mirroring backend.machines
 * Loop through backend.zones, asking each one for all of its zones