Improve FieldLayout help popup appearance

* Add a class to both the button and the help text
* Make the help text a div instead of a span
* Remove the title attribute, was causing huge tooltips
* Don't add an empty <div> if there's no help text,
  just add nothing instead
* Add some padding in the popup
* Float all the help icons right

Change-Id: I974c7c3c92886249cf1bfb4194a3a6c9814c0395