Currently, an empty element was appended to hero-icon template to hold
the label even if this didn't exist, causing some vertical bottom
spacing.
We use Polymer's `dom-if` to conditional show this element. The label
was changed to be passed as an HTML property since it seems to be really
difficult to access the inner HTML of a polymer element programatically.
This screen informs the user that the burn has completed and that the
drive can be ejected directly.
It also provides a button to burn another image without exitting the
application.
Fixes: https://github.com/resin-io/herostratus/issues/41
