Bug 936639 - [Messages][Drafts] Create Drafts object + tests

- Drafts Singleton
    - Adds, deletes, clears Draft objects
    - Fetches all Draft objects for a thread id
- Drafts.List object
    - Array-like holder for Draft objects
    - has readonly accessor for length
    - has forEach to run callback on all contained Draft objects
- Draft object
    - standalone class to describe fields for a Draft instance
- Tests
    - Drafts: unit tests for adding, deleting, clearing, fetching by id
    - Drafts.List: unit tests for length, forEach
    - Draft: unit tests for new Draft from empty object and from existing Draft
- Issues
    - Not sure if recipients should be stored as Array or String