feat(aio): sidenav headers should be focusable buttons

Sidenav headers had been anchors w/o hrefs. These can’t take focus which makes you can’t navigate through them with keyboard. For a11y purposes, this PR turns them into buttons.

aio/e2e/app.e2e-spec.ts

@@ -5,8 +5,8 @@ describe('site App', function() {
   let page: SitePage;
 
   beforeEach(() => {
+    SitePage.setWindowWidth(1050);   // Make the window wide enough to show the SideNav side-by-side.
     page = new SitePage();
-    page.setWindowWidth(1050);   // Make the window wide enough to show the SideNav side-by-side.
     page.navigateTo();
   });