commento.js: add hidden label to textarea (for accessibility) and rel="noreferrer noopener" to comment.io link
Labels are necessary for users that depend on a screen reader. This merge request adds the label and hides it so the UI remains the same (it is till readable by screen readers).
I also added a rel="noopener noreferrer" attribute to the https://comment.io link ('Powered by Commento') since it is an external link.
Both changes improve the Lighthouse score of sites using Commento.
Edited by Pablo Berganza