seo: update JSON-LD site metadata.

* Add the Danbooru logo and social media links to the Organization data.
  This is to make Danbooru eligible for Google's knowledge panel ([1], [2]).
* Only include this metadata on the frontpage as per Google's recommendations ([3]).

[1]: https://support.google.com/knowledgepanel/answer/9163198
[2], https://developers.google.com/search/docs/data-types/logo
[3]: https://developers.google.com/search/docs/data-types/sitelinks-searchbox

app/helpers/seo_helper.rb

@@ -0,0 +1,56 @@
+# https://yoast.com/structured-data-schema-ultimate-guide/
+# https://technicalseo.com/tools/schema-markup-generator/
+# https://developers.google.com/search/docs/data-types/sitelinks-searchbox
+# https://developers.google.com/search/docs/data-types/logo
+# https://search.google.com/structured-data/testing-tool/u/0/
+# https://search.google.com/test/rich-results
+# https://schema.org/Organization
+# https://schema.org/WebSite
+
+module SeoHelper
+  def site_description
+    "#{Danbooru.config.canonical_app_name} is the original anime image booru. Search millions of anime pictures categorized by thousands of tags."
+  end
+
+  def json_ld_website_data
+    urls = [
+      Danbooru.config.twitter_url,
+      Danbooru.config.discord_server_url,
+      Danbooru.config.source_code_url,
+      "https://en.wikipedia.org/wiki/Danbooru"
+    ].compact
+
+    json_ld_tag({
+      "@context": "https://schema.org",
+      "@graph": [
+        {
+          "@type": "Organization",
+          "@id": root_url(anchor: "organization", host: Danbooru.config.hostname),
+          url: root_url(host: Danbooru.config.hostname),
+          name: Danbooru.config.app_name,
+          logo: "#{root_url(host: Danbooru.config.hostname)}images/danbooru-logo-500x500.png",
+          sameAs: urls
+        },
+        {
+          "@type": "WebSite",
+          "@id": root_url(anchor: "website", host: Danbooru.config.hostname),
+          "url": root_url(host: Danbooru.config.hostname),
+          "name": Danbooru.config.app_name,
+          "description": site_description,
+          "publisher": {
+            "@id": root_url(anchor: "organization", host: Danbooru.config.hostname),
+          },
+          "potentialAction": [{
+            "@type": "SearchAction",
+            "target": "#{posts_url(host: Danbooru.config.hostname)}?tags={search_term_string}",
+            "query-input": "required name=search_term_string"
+          }]
+        }
+      ]
+    })
+  end
+
+  def json_ld_tag(data)
+    tag.script(data.to_json.html_safe, type: "application/ld+json")
+  end
+end