Adjust render_new_comments_node_links to fix issues

In tracker-history.js and tracker-history.es6.js there are several cases where previousSibling is being used and that causes issues with whitespace.

Example: Controller \Drupal\tracker\Controller\TrackerUserTab::getContent

This renders a table with 5 td's per row and the JavaScript in tracker-history.js processes the indicators for new comments like this:

  function processNewRepliesIndicators($placeholders) {
    var placeholdersToUpdate = {};
    $placeholders.each(function (index, placeholder) {
      var timestamp = parseInt(placeholder.getAttribute('data-history-node-last-comment-timestamp'), 10);
      var nodeID = placeholder.previousSibling.previousSibling.getAttribute('data-history-node-id');
      var lastViewTimestamp = Drupal.history.getLastRead(nodeID);

      if (timestamp > lastViewTimestamp) {
        placeholdersToUpdate[nodeID] = placeholder;
      }
    });

    var nodeIDs = Object.keys(placeholdersToUpdate);
    if (nodeIDs.length === 0) {
      return;
    }
    $.ajax({
      url: Drupal.url('comments/render_new_comments_node_links'),
      type: 'POST',
      data: { 'node_ids[]': nodeIDs },
      dataType: 'json',
      success: function success(results) {
        for (var nodeID in results) {
          if (results.hasOwnProperty(nodeID) && placeholdersToUpdate.hasOwnProperty(nodeID)) {
            var url = results[nodeID].first_new_comment_link;
            var text = Drupal.formatPlural(results[nodeID].new_comment_count, '1 new', '@count new');
            $(placeholdersToUpdate[nodeID]).append('<br /><a href="' + url + '">' + text + '</a>');
          }
        }
      }
    });
  }

The 2 instances of previousSibling are returning the wrong td because the first previousSibling returns a text node and not the previous td node. As a result, the nodeID is NULL and then nodeIDs contains one element NULL which causes the ajax to being fired and that causes trouble in the backend.

I guess all instances of previousSibling should be replaced with previousElementSibling to securely avoid that. I'll submit a patch in a second.