Bug 959196 - [Messages][Drafts] Don't auto-save new draft when hiding app and then draft is discarded r=rwaldron

- `thread_ui.js`
    - If `MessageManager` is not already holding onto a draft when a brand new is auto-saved, then set it. this way preservation/discarding works as expected
- Tests
    - `thread_ui_test.js`
        - Test that `MessageManager.draft` is properly set if it didn't exist before saving a draft