Fix icons in turnable template.

aleksis/core/templates/core/turnable.html

@@ -11,10 +11,10 @@
    </div>
    <div class="btn-group" role="group" aria-label="URL actions">
      <a href="{{ url_prev }}" class="btn btn-dark">
-       <span class="mdi mdi-chevron-left"></span>
+       <i class="material-icons">chevron_left</i>
      </a>
      <a href="{{ url_next }}" class="btn btn-dark">
-       <span class="mdi mdi-chevron-right"></span>
+       <i class="material-icons">chevron_right</i>
      </a>
    </div>
   </div>