Bug 936639 - [Messages][Drafts] Create Drafts module/constructor + tests

- Drafts Singleton
    - Adds, deletes, clears Draft objects
    - Loads and stores Draft objects from storage
    - Fetches all Draft objects for a thread id
- Drafts.List object
    - Array-list 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, deletion, clearing, storing, loading, fetching by id
    - Draft.List: unit tests for length, forEach
    - Draft: no unit tests yet