Using the .html extension for upgrade.release_notes makes the comment for
translators in <!-- --> appear as a comment for the string in the PO file.

  - Using an <a href=""> link like we do for the ISO image was not possible
    because the link needed to be a full ikiwiki link in order not to work with
    relative links.
  - So the /inc file includes an ikiwiki link that also needs to be translated.
  - The downside is that this will have to be handle by the release process not
    to break the translation on each release...