social-icons: add rel=me

* rel=me links allow cross-linking of user's identities, improving SEO

layouts/partials/social_icons.html

@@ -1,6 +1,6 @@
 <div class="social-icons">
     {{- range . -}}
-    <a href="{{ .url }}" target="_blank" rel="noopener noreferrer"
+    <a href="{{ .url }}" target="_blank" rel="noopener noreferrer" rel="me"
         title="{{ .name | humanize }}">{{ partial "svg.html" . }}</a>
     {{- end -}}
-</div>
+</div>