FEATURE: Add /search discovery

The opensearch.xml results in a "site search engine" being added to
Chrome, while the sitelinks search tag results in "Search this website"
being added to Google Search.

app/helpers/application_helper.rb

@@ -157,6 +157,20 @@ module ApplicationHelper
     result.join("\n")
   end
 
+  def render_sitelinks_search_tag
+    json = {
+      '@context': 'http://schema.org',
+      '@type': 'WebSite',
+      url: Discourse.base_url,
+      potentialAction: {
+        '@type': 'SearchAction',
+        target: "#{Discourse.base_url}/search?q={search_term_string}",
+        'query-input': 'required name=search_term_string',
+      }
+    }
+    content_tag(:script, MultiJson.dump(json).html_safe, type: 'application/ld+json'.freeze)
+  end
+
   def application_logo_url
     @application_logo_url ||= (mobile_view? && SiteSetting.mobile_logo_url) || SiteSetting.logo_url
   end