Bug 903960 - [SMS] Switching to message view when selecting left side of the message

- Fix
    - Fixed by moving the `.pack-checkbox` `<li>` over to the left, to completely cover the underlying `<a>` tag that was launching the message view.  Then, compensated for the move by moving the contained `<span>` tag containing the checkbox graphic over to the right again
- Tests
    - Manual in both desktop and on Unagi device using `m-c` gecko
    - Need any automated tests?