added favicon (facebookarchive#1871)

Summary:
Fixed facebookarchive#1857 by adding a favicon. The color scheme of the icon needs to be thought about though since it is not visible when the tab is active because of the white color.

On chrome:
![image](https://user-images.githubusercontent.com/1450819/45538271-39089f80-b824-11e8-8553-5902b11a53bb.png)

Looks okay when the tab is inactive, but when it is active, the logo is not visible:
![image](https://user-images.githubusercontent.com/1450819/45538295-4b82d900-b824-11e8-9b27-339dc2c797d3.png)
Pull Request resolved: facebookarchive#1871

Differential Revision: D9829164

fbshipit-source-id: 99c7f4d805b7da2713c3596992b39c125b63b221

Author: saranshkataria

website/siteConfig.js

@@ -21,6 +21,7 @@ const siteConfig = {
     }
   ],
   headerIcon: 'img/draftjs-logo.svg',
+  favicon: 'img/draftjs-logo.ico',
   /* colors for website */
   colors: {
     primaryColor: '#3B3738',