routes: add a new 404 page.

* Fix a bug where non-GET 404 requests weren't handled.
* Fix a bug where non-HTML 404 requests weren't handled.
* Show a random image from a specified pool on the 404 page.

config/danbooru_default_config.rb

@@ -281,6 +281,11 @@ module Danbooru
       restricted_tags + %w[censored condom nipples nude penis pussy sexually_suggestive]
     end
 
+    # If present, the 404 page will show a random post from this pool.
+    def page_not_found_pool_id
+      nil
+    end
+
     # Tags that are only visible to Gold+ users.
     def restricted_tags
       []