Update Site list in the editor to use the `wpcloud-station/v1/sites` endpoint

[jhnstn]

Currently, the site-list block uses the custom post type endpoint wp/v2/wpcloud_site . Ideally it would use the new endpoint wpcloud-station/v1/sites so we can remove the wp/v2/wpcloud_site routes from the Sites controller.

The site list block is mostly copy/paste from the core post query block. Most of the logic in there should be simplified to make it easier to dispatch the new REST endpoints.