header: add sitelinks search box

[jlchereau]

See:

• https://developers.google.com/structured-data/slsb-overview
• http://www.opensearch.org/Specifications/OpenSearch/1.1

<link rel="search" href="http://www.macworld.co.uk/search/">
<link rel="search" type="application/opensearchdescription+xml" title="Macworld UK Search" href="/assets/search.osdx">

and

<script type="application/ld+json">
{
"@context" : "http://schema.org",
"@type" : "Organization",
"name" : "Macworld UK",
"url" : "http://www.macworld.co.uk/",
"potentialAction": {
"@type": "SearchAction",
"target": "http://www.macworld.co.uk/search/?q={search}",
"query-input": "required name=search"
},
"sameAs" : [
"https://www.facebook.com/MacworldUK",
"https://twitter.com/MacworldUK",
"https://plus.google.com/102391000274002633151",
"http://www.youtube.com/channel/UCT7WejN8j_nGiSfwK8TnBcw"
]
}
</script>