Add the background image to the authentication email input.

* Update the border of the input boxes to match the mocks.

issue #3410
Use CSS to hide the placeholder text on mobile. Much nicer.
Shrink the amount of top/bottom padding so the Persona image looks better on Chrome for Android.

Add a comment about why Android ICS default browser causes the background image to disappear.
Merge pull request #3421 from mozilla/issue_3410_email_input_bg

Add the background image to the authentication email input.