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
       []

config/routes.rb

@@ -321,5 +321,5 @@ Rails.application.routes.draw do
   get "/mock/iqdbs/similar" => "mock_services#iqdbs_similar", as: "mock_iqdbs_similar"
   post "/mock/iqdbs/similar" => "mock_services#iqdbs_similar"
 
-  get "*other", :to => "static#not_found"
+  match "*other", to: "static#not_found", via: :all
 end