Bug 951331 - [Messages][Drafts] MMS indicator missing for MMS drafts in existing (non-threadless) threads

- `thread_list_ui.js
    - set the lastMessageType to the draft's message type before applying a style to the DOM element
- Tests
    - `thread_list_ui_test.js`
        - Refactored the 'Correctly displayed content' test suite
        - Added test to check that `li.dataset.lastMessageType` was properly set if draft is of type 'mms'
        - Manually tested on device