Add rel=related link header to Node GET and HEAD responses

[dannylamb]

Add a rel=related link header to Node GET responses, pointing to all referenced entiies. The 'title' of the link header should be the name of the entity reference field. This way, we can navigate the repository using link headers, finding derivatives or parent collections, etc...

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"'],
  ];
}