Bug 949137 - [Messages][Drafts] Composer doesn't clear after sending a new draft

-`thread_ui.js`
    - instead of trying to get a thread id, just delete the draft if it existed in `MessageManager.draft`
    - clear composer
    - update thread list

-`thread_ui_test.js`
    - test that the composer clears, draft is deleted, threadlist is updated and `MessageManager.draft` is set to null