Bug 936645 - [Messages][Drafts] Add banner message for saved draft

- `index.html`
    - Added markup for banner based on [http://buildingfirefoxos.com/building-blocks/status.html](http://buildingfirefoxos.com/building-blocks/status.html)
- `thread_list_ui.js`
    - Added a draft saved duration constant for 5 seconds
    - Added a pointer to the draft saved banner markup
    - Added a draft saved function that shows the banner when called
- `thread_ui.js`
    - Calls `ThreadListUI.draftSaved` after a draft is saved
- `sms.en-US.properties`
    - Added locale string for banner => "Message draft saved"
- Tests
    - Unit
        - `thread_list_ui_test.js`
            - Check that banner is hidden, shown for 5 seconds, and hidden again during `draftSaved()`
        - `thread_ui_test.js`
            - Check that `draftSaved()` is called after draft is saved
    - Manual
        - Save a draft, look for banner
        - Replace a draft, look for banner