Add facebook login button.

Adds basic [Facebook Login](https://developers.facebook.com/docs/plugins/login-button/) button to the homepage. Kept under a separate branch since the following must be added before merging with `master`:

- Passing `authenticated_p` to index GET handler from FB event;
- Hiding 'hamburger' on homepage when not authenticated;
- Wrapping other pages in a [call to the FB API to check for login](https://developers.facebook.com/docs/reference/javascript/FB.getLoginStatus/);