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.

app/controllers/static_controller.rb

@@ -1,4 +1,6 @@
 class StaticController < ApplicationController
+  respond_to :html, :json, :xml
+
   def privacy_policy
   end
 
@@ -6,7 +8,11 @@ class StaticController < ApplicationController
   end
 
   def not_found
-    render plain: "not found", status: :not_found
+    @pool = Pool.find(Danbooru.config.page_not_found_pool_id) if Danbooru.config.page_not_found_pool_id.present?
+    @post = @pool.posts.sample if @pool.present?
+    @artist = @post.tags.select(&:artist?).first if @post.present?
+
+    render_error_page(404, nil, template: "static/not_found", message: "Page not found")
   end
 
   def error
@@ -38,7 +44,7 @@ class StaticController < ApplicationController
       @search = { is_deleted: "false" }
     when "posts"
       @relation = Post.order(id: :asc)
-      @serach = {}
+      @search = {}
     when "tags"
       @relation = Tag.nonempty
       @search = {}