Fix #4425: Add Danbooru Discord server prominently

* Add Discord, Twitter, and Github links to site footer.
* Remove "Desktop mode" and "Keyboard shortcuts" links from footer.
  Users can use the desktop site option in their browser to switch
  between desktop and mobile mode.
* Remove git hash from footer. Users can hover over the Github logo to
  see the hash instead.

config/danbooru_default_config.rb

@@ -425,6 +425,11 @@ module Danbooru
       end
     end
 
+    # The url of the Discord server associated with this site.
+    def discord_server_url
+      nil
+    end
+
     # The twitter username associated with this site (username only, don't include the @-sign).
     def twitter_username
       nil