ENYO-3708: fix `preview` following layout change

- use `/preview.html` instead of `/preview/index.html`
- Preview source now interleaved with Ares source (needed because some
  of the Ares built-in code is used as a library)

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>