Move deletion of a site to background a process

Currently sites are deleted asynchronously, but can remain visible in the app while the deletion is underway. Depending on the amount of data to be deleted, this can take some time.

It would be nice if a site would have a similar 'deleted' flag as jobs and builds have and that a background process will handle the deletion of the data.

While the site is still available, it should only be listed in the administration section of the app where an admin can only cancel the deletion by clearing the marker. All other actions should be disabled.