Add rel="me" to footer links for web sign-in

This builds on the microformats enhancements contributed by @dltj in mmistakes#3052.

Adding `rel="me"` to footer links [enables web sign-in](https://indieweb.org/How_to_set_up_web_sign-in_on_your_own_domain) or [RelMeAuth](https://microformats.org/wiki/RelMeAuth), so you can use your domain to sign in to other sites.

In mmistakes#3052, the `rel="me"` attributes were added to the author profile, but that only appears in post sidebars, and web sign-in requires these profile links to be present on the home page.

_includes/footer.html

@@ -7,7 +7,7 @@
     {% if site.footer.links %}
       {% for link in site.footer.links %}
         {% if link.label and link.url %}
-          <li><a href="{{ link.url }}" rel="nofollow noopener noreferrer"><i class="{{ link.icon | default: 'fas fa-link' }}" aria-hidden="true"></i> {{ link.label }}</a></li>
+          <li><a href="{{ link.url }}" rel="nofollow noopener noreferrer me"><i class="{{ link.icon | default: 'fas fa-link' }}" aria-hidden="true"></i> {{ link.label }}</a></li>
         {% endif %}
       {% endfor %}
     {% endif %}