Summary:
**Summary**

After internal discussion and approval by the legal team, we decided to align Draft.js with other Facebook open source projects [recently re-licensed under MIT](https://code.fb.com/web/relicensing-react-jest-flow-and-immutable-js/).

This PR removes the patents grant and re-licenses all Draft.js source files under the MIT license.

**Test Plan**

`yarn lint`
Manual code inspection
Pull Request resolved: https://github.com/facebook/draft-js/pull/1967

Differential Revision: D13572529

fbshipit-source-id: f59982148515fdc7a2f697c92e1939e211b6125c