Improve accessibility of navigation menu button

Add `type` and label text.

Fixes #1099

_includes/masthead.html

@@ -13,7 +13,10 @@
             <li class="masthead__menu-item"><a href="{{ domain }}{{ link.url }}">{{ link.title }}</a></li>
           {% endfor %}
         </ul>
-        <button><div class="navicon"></div></button>
+        <button type="button">
+          <span class="visually-hidden">{{ site.data.ui-text[site.locale].menu_label | default: "Toggle Menu" }}</span>
+          <div class="navicon"></div>
+        </button>
         <ul class="hidden-links hidden"></ul>
       </nav>
     </div>