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

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