Bug 920164 - [MMS] Display the subject of an MMS, if present

- `thread_ui.js`
    - added has-subject CSS class if message type is mms
    - added subject to the message DOM template if mms and has subject
- `index.html`
    - added subject h1 and template item
- `sms.css`
    - styling for mms message bubble with subject
    - styling for subject h1
- `thread_ui_test.js`
    - added unit test to check against template interpolation with subject field for mms