navbar: Hide mobile toggle button if there are no menu items (#307)

layouts/partials/navbar.html

@@ -3,6 +3,7 @@
 
     <!-- Brand and toggle get grouped for better mobile display -->
     <div class="navbar-header">
+      {{ if or .Site.Menus.main .IsTranslated }}
       <button type="button" class="navbar-toggle collapsed" data-toggle="collapse"
               data-target=".navbar-collapse" aria-expanded="false">
         <span class="sr-only">{{ i18n "toggle_navigation" }}</span>
@@ -10,6 +11,7 @@
         <span class="icon-bar"></span>
         <span class="icon-bar"></span>
       </button>
+      {{ end }}
       <a class="navbar-brand" href="{{ "/" | relLangURL }}">{{ .Site.Title }}</a>
     </div>