Replace sidebar with app-drawer element

This PR add's the app-drawer element from the [app-widgets project](https://github.com/abarth/app-widgets). I've had to [tweak some things](https://github.com/Polymer/docs/compare/add-app-drawer?expand=1#diff-3211412c06d2feb3815cd58b4034a7e7R87) in the app-drawer.html file because our site has 3 states for the sidebar: open/closed on mobile and fixed left for desktop.

/cc @ebidel @addyosmani @abarth for review