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

app/views/layouts/default.html.erb

@@ -19,42 +19,12 @@
   <% if CurrentUser.user.custom_style.present? && params.fetch(:css, "true").truthy?  %>
     <%= stylesheet_link_tag custom_style_users_path(md5: Digest::MD5.hexdigest(CurrentUser.user.custom_style)), media: "screen" %>
   <% end %>
-  <% if Danbooru.config.twitter_username.present? %>
-    <script type="application/ld+json">
-      {
-        "@context" : "http://schema.org",
-        "@type" : "Organization",
-        "name" : "<%= Danbooru.config.app_name %>",
-        "url" : "<%= root_url %>",
-        "sameAs" : [
-          "https://twitter.com/<%= Danbooru.config.twitter_username %>"
-        ]
-      }
-    </script>
-  <% end %>
-  <script type="application/ld+json">
-  {
-    "@context": "http://schema.org",
-    "@type": "WebSite",
-    "url" : "<%= root_url %>",
-    "potentialAction": [{
-      "@type": "SearchAction",
-      "target": "<%= posts_url %>?tags={search_term_string}",
-      "query-input": "required name=search_term_string"
-    }]
-  }
-  </script>
-  <script type="application/ld+json">
-  {
-    "@context": "http://schema.org",
-    "@type": "WebSite",
-    "name": "<%= Danbooru.config.app_name %>",
-    "alternateName": "Anime Image Board",
-    "url" : "<%= root_url %>"
-  }
-  </script>
-  <%= tag.meta name: "description", content: meta_description %>
 
+  <% if current_page?(root_url) %>
+    <%= json_ld_website_data %>
+  <% end %>
+
+  <%= tag.meta name: "description", content: meta_description %>
   <%= tag.meta property: "og:type", content: "website" %>
   <%= tag.meta property: "og:site_name", content: Danbooru.config.app_name %>
   <%= tag.meta property: "og:title", content: page_title %>

app/views/posts/partials/index/_seo_meta_tags.html.erb

@@ -1,6 +1,6 @@
 <% if @post_set.query.is_empty_search? %>
   <% page_title("#{Danbooru.config.app_name}: Anime Image Board", suffix: nil) %>
-  <% meta_description("#{Danbooru.config.canonical_app_name} is the original anime image 'booru. Find over 3.75 million anime pictures categorized by over 100 million tags.") %>
+  <% meta_description site_description %>
 
   <% atom_feed_tag "Posts", posts_url(format: :atom) %>
 <% else %>

config/danbooru_default_config.rb

@@ -355,6 +355,11 @@ module Danbooru
       nil
     end
 
+    def twitter_url
+      return nil unless Danbooru.config.twitter_username.present?
+      "https://twitter.com/#{Danbooru.config.twitter_username}"
+    end
+
     # include essential tags in image urls (requires nginx/apache rewrites)
     def enable_seo_post_urls
       false