Expose described File as link header for Media Entities

[dannylamb]

Every time a media is viewed, check its fields for a referenced file or image (currently field_file or field_iage) and expose the file's URL as a link header.

You have access to the media and can add link headers in hook_media_view_alter like so:

function islandora_media_view_alter(&$build, EntityInterface $media, EntityViewDisplay $display) {
  $build['#attached']['http_header'] = [
    ['Link', '<http://islandora.ca>; rel="awesome"'],
  ];
}

Use "describes" for the link header's rel value.

[dannylamb]

Duplicate of #721