Bug 957708 - [Messages][Drafts] Draft status indicator not accurate when app loads

- `thread_list_ui.js`
    - after `Drafts.request` returns, go through all the rendered threads, and for those that have drafts, update them so they correctly show the draft icon
- Tests:
    - `thread_list_ui_test.js`
        - verify that `updateThread` is called on existing threads that have drafts