Visual-only collapsing of header and facets.

- Removes need for "Toggle navigation" label.
- Changes how Bootstrap's .collapse works.

themes/bootstrap3/less/components/accessibility.less

@@ -19,6 +19,37 @@ a {
 }
 
 .sr-only {
+  color: #000;
   background-color: #fff;
+}
+
+/**
+ * instead of display: none, keep things sr accessible
+ * https://tailwindcss.com/docs/screen-readers/
+ */
+.collapse.collapse:not(.in) {
+  position: absolute;
+  display: block;
+  width: 1px;
+  height: 1px;
+  padding: 0;
+  margin: -1px;
   color: #000;
+  background-color: #fff;
+  overflow: hidden;
+  clip: rect(0, 0, 0, 0);
+  white-space: nowrap;
+  border-width: 0;
+}
+@media (min-width: 768px) {
+  .navbar-collapse.collapse:not(.in) {
+    position: static;
+    width: auto;
+    height: auto;
+    padding: 0;
+    margin: 0;
+    overflow: visible;
+    clip: auto;
+    white-space: normal;
+  }
 }

themes/bootstrap3/templates/header.phtml

 <?php $account = $this->auth()->getManager(); ?>
 <div class="banner container navbar">
   <div class="navbar-header">
-    <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#header-collapse">
-      <span class="sr-only">Toggle navigation</span>
-      <i class="fa fa-bars" aria-hidden="true"></i>
+    <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#header-collapse" aria-hidden="true">
+      <i class="fa fa-bars"></i>
     </button>
     <a class="navbar-brand lang-<?=$this->layout()->userLang ?>" href="<?=$this->url('home')?>">VuFind</a>
   </div>