Added missing ref attribute (#1959)

As suggested by @mmistakes in mmistakes/minimal-mistakes#1948 (comment)

Author: kulbhushanchand

_includes/author-profile.html

@@ -51,7 +51,7 @@ <h3 class="author__name" itemprop="name">{{ author.name }}</h3>
       {% if author.links %}
         {% for link in author.links %}
           {% if link.label and link.url %}
-            <li><a href="{{ link.url }}"><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"><i class="{{ link.icon | default: 'fas fa-link' }}" aria-hidden="true"></i> {{ link.label }}</a></li>
           {% endif %}
         {% endfor %}
       {% endif %}