Move search role onto input rather than the form

From Léonie:

> [Search is] being said three times, but it isn't the toggle that's doing
> it. It's the search role on the <form> element, the text label, and the
> search type of the <input> element.
>
> For the moment the triple announcement is a fairly edge case, but as
> more screen readers start to support both ARIA and HTML5 it'll become
> much more common. Best bet is to reduce the announcements down to two
> by applying the search role to the <input> element.
Merge pull request #218 from alphagov/search

Move search role onto input rather than the form