Updates favicons and launcher icons with Draftjs logo (#1872)

Summary:
**Summary**

Updates icons and launcher icons to use the Draftjs logo on a maroon backdrop instead of the "D" letter.

**Test Plan**

* `yarn build && cd website && yarn start`
* Verify the favicon is the new icon with the Draftjs logo

<img width="170" alt="screen shot 2018-09-15 at 2 06 40 pm" src="https://user-images.githubusercontent.com/860099/45586618-1e9cf780-b8f2-11e8-8b7a-ebc20af5e5c4.png">

_**Note**: apparently, Docusaurus doesn't seem to use the apple and android launcher icons on this config. I'll try and look into that on a separate PR (will add support into Docusaurus itself if needed)_
Pull Request resolved: #1872

Differential Revision: D9846099

fbshipit-source-id: 347ae57270eebaeedadd324af9ca78a74421713c

Author: Claudio Procida