Make html email look prettier on mobile device

Add a media query style for mobile device so the email doesn't generate
a horizontal bar.  Gmail strips out all style, that means we can't apply
the mobile style to Gmail.  Luckily, Gmail on mobile device browser
automatically fit into the entire mobile screen

bug: 53057

Change-Id: Ia4350669db2e81ee44d5b53d7cece6fcd8839e7a