Search component hint does not allow HTML to be passed in

[chrispymm]

Standard GOV.UK design system text inputs allow a html option to be passed instead of text for the hint text. The search component does not allow this, and passing a hint with an html key simply renders an empty hint element.

For consistency with the main design system and ease of use for developers we should follow GOV.UK conventions and match where possible the options accepted by the GOV.UK text input.