Use proper semantic elements for subnav

source/_patterns/02-organisms/00-global/subnav.mustache

 <div class="__js-root" data-app="Subnav">
   <ui-subnav inline-template>
     <ui-subnav-view-provider :initial="{regions: false, calendar: false}" v-slot="{ showView, toggleView }">
-      <div class="subnav">
+      <nav class="subnav">
         <div class="container container--wide">
           <div class="flex">
             <button
@@ -41,8 +41,8 @@
             </button>
           </div>
         </div>
-      </div>
-      <div class="subnav-aside">
+      </nav>
+      <aside class="subnav-aside">
         <div class="subnav-aside__item" :class="{'subnav-aside__item--visible': showView('regions')}">
           <a @click="toggleView('regions')" class="subnav-aside__close"><i class="fas fa-times"></i></a>
           <ui-region-map class="container container--default" />
@@ -53,7 +53,7 @@
             {{> molecules-calendar }}
           </div>
         </div>
-      </div>
+      </aside>
     </ui-subnav-view-provider>
   </ui-subnav>
 </div>