Add a rel=alternate link header to Node GET responses

[dannylamb]

Add a rel=alternate link header to Node GET responses for each Media that is not a Preservation Master (e.g. derivatives).

You have access to the node and can add link headers in hook_node_view_alter like so:

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

[dannylamb]

Closing since #716 takes care of this more generically.