Bug 964862 - [Messages][Drafts][Refactoring] Move MessageManager.draft to ThreadUI.draft

- Fix
    - Add `draft` property to `ThreadUI`
    - Remove 'draft' property from `MessageManager`
    - Change instances of `MessageManager.draft` to `ThreadUI.draft`
    - Fix globals
- Tests
    - Fix `compose_test.js` to use a true `Draft` in `ThreadUI.draft` to fix CI tests
    - Fix `message_manager_test.js` to use a null `ThreadUI.draft` to fix CI tests
    - Change instances of 'MessageManager.draft` to `ThreadUI.draft` in all unit tests
    - Fix globals