Tabs plugin accessibility

Added support for `aria-expanded` toggling via JavaScript. Added
`aria-controls`, `aria-expanded`, `aria-labelledby` to tabs docs.

Added `aria-expanded` unit test for the tabs plugin.

See also #13554.

Closes #14154 by merging it.

js/tab.js

@@ -71,9 +71,15 @@
       $active
         .removeClass('active')
         .find('> .dropdown-menu > .active')
-        .removeClass('active')
+          .removeClass('active')
+        .end()
+        .find('[data-toggle="tab"]')
+          .attr('aria-expanded', false)
 
-      element.addClass('active')
+      element
+        .addClass('active')
+        .find('[data-toggle="tab"]')
+          .attr('aria-expanded', true)
 
       if (transition) {
         element[0].offsetWidth // reflow for transition
@@ -83,7 +89,12 @@
       }
 
       if (element.parent('.dropdown-menu')) {
-        element.closest('li.dropdown').addClass('active')
+        element
+          .closest('li.dropdown')
+            .addClass('active')
+          .end()
+          .find('[data-toggle="tab"]')
+            .attr('aria-expanded', true)
       }
 
       callback && callback()