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

- `drafts.js`
    - added a `forEach` that calls a callback on the latest draft per non-null `threadId` and all the drafts with a null `threadId`
- `thread_list_ui.js`
    - after `Drafts.request` returns, go through all the drafts, and for those that have a valid `threadId` and there is a rendered thread element for that `threadId`, update them so they mark themselves appropriately as having drafts
- Tests:
    - `drafts.js`
        - verify that a callback is called on all latest drafts per valid `threadId` and all null `threadId` drafts
    - `thread_list_ui_test.js`
        - verify that `updateThread` is called on existing threads that have drafts
    - Tested manually on device