Fix preview of theming

Pull request #5429 made cached SCSS files depend on a hash of the base
URL, so the "/css/core/server.css" file does no longer exist; as the
file can not be loaded the "Loading preview" message is never removed
and the "Saved" message is never shown.

As it now depends on the hash of the base URL the file to be reloaded
can no longer be hardcoded, so the full URL to the "server.css" file
that has to be reloaded (if any) is now got from the DataResponse
provided by the controller.

Fixes #5975

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>