feat(back-to-top): improve accessibility of back-to-top

- Remove `role=button`, since back-to-top is a navigating element
- Add `href=#top` so it is percieved as a link and becomes tabbable
- Add aria-label

[Finishes #100047042]

Signed-off-by: Geoff Pleiss <gpleiss@pivotal.io>

src/pivotal-ui-react/back-to-top/back-to-top.js

@@ -59,7 +59,7 @@ var BackToTop = React.createClass({
         style: {display: 'inline', opacity: this.animate('opacity', visible ? 1 : 0, BackToTop.FADE_DURATION)}
       }
     );
-    return <a {...props} role='button' onClick={this.scrollToTop}/>;
+    return <a {...props} onClick={this.scrollToTop} href="#top" aria-label="Back to top" />;
   }
 });