Revert "Move search role onto input rather than the form"

This reverts commit a11375899bbd3b4a123ee53ded8d8c8020b665f9.

From Léonie:

> The problem is that ARIA overrides the native semantics of HTML. In this
> case the ARIA search role overrides the native role of the input
> element.  This could mean that some ATs (screen readers etc.) might not
> report the presence of the input correctly, because all they're aware of
> is the search landmark (in ARIA terms a landmark is a region containing
> a group of related elements).

Untill we have a better solution we should undo the changes we made.
Merge pull request #220 from alphagov/search-2

Revert "Move search role onto input rather than the form"