This helps superusers understand better what, exactly, is going on in the registry
Return an empty body on API requests with status code 204, which means "No content". Incorrect 'Deleted' responses were being returned after successful DELETE operations despite the "No Content" definition of 204.
Fixes #2133
Adds basic labels support to the registry code (V2), and the API. Note that this does not yet add any UI related support.