Improve API readbility in RestControllers

[bglamadrid]

Using ResponseEntity is a great way to write controllers down as if they were API documentation, with declared HTTP response codes.

[bglamadrid]

It is a cleaner practice to annotate with @ResponseStatus and only use ResponseEntity for uncommon status codes that require a bit more complex preparation, such as the 301 SEE_OTHER code that should include a Location header with a valid URI.