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/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 %>