Refocus table of contents overlay trigger on close

When using the dialog with a keyboard, focus is
not set when you close it so the browser has to
guess what to focus. This is usually on the
document itself or the main content area, both of
which are confusing to users.

This focuses the button that opens the dialog when
it closes, as per WAI ARIA advice from the W3C:

https://www.w3.org/WAI/ARIA/apg/patterns/dialog-modal/

Author: tombye

lib/assets/javascripts/_modules/table-of-contents.js

@@ -113,6 +113,7 @@
       $html.removeClass('toc-open')
 
       toggleBackgroundVisiblity(true)
+      $openButton.focus()
       updateAriaAttributes()
     }
 

spec/javascripts/table-of-contents-spec.js

@@ -197,7 +197,8 @@ describe('Table of contents', function () {
   })
 
   describe('if the close button is clicked', function () {
-    var clickEvt
+    var openClickEvt
+    var closeClickEvt
 
     beforeEach(function () {
       $html.addClass('mobile-size')
@@ -206,21 +207,28 @@ describe('Table of contents', function () {
       module.start($toc)
 
       // tocIsVisible = false // controls what $toc.is(':visible') returns, which will be controlled by CSS in a web page
-      clickEvt = new $.Event('click')
-      $closeButton.trigger(clickEvt)
+      openClickEvt = new $.Event('click')
+      closeClickEvt = new $.Event('click')
+
+      $openButton.trigger(openClickEvt)
+      $closeButton.trigger(closeClickEvt)
     })
 
     afterEach(function () {
       $html.removeClass('mobile-size')
     })
 
     it('the click event should be cancelled', function () {
-      expect(clickEvt.isDefaultPrevented()).toBe(true)
+      expect(closeClickEvt.isDefaultPrevented()).toBe(true)
     })
 
     it('the table of contents should be hidden', function () {
       expect($toc.attr('aria-hidden')).toEqual('true')
     })
+
+    it('the button that triggered the dialog is refocused', function () {
+      expect(document.activeElement).toBe($openButton.get(0))
+    })
   })
 
   it('on mobile-size screens, when the table of contents is open and the escape key is activated, the table of contents should be hidden', function () {