api: refactor default options for xml responses.

In xml responses, if the result is an empty array we want the response
to look like this:

   <posts type="array"/>

not like this (the default):

   <nil-classes type="array"/>

This refactors controllers so that this is done automatically instead of
having to manually call `@things.to_xml(root: "things")` everywhere. We
do this by overriding the behavior of `respond_with` in `ApplicationResponder`
to set the `root` option by default in xml responses.

config/application.rb

@@ -8,6 +8,9 @@ require_relative "danbooru_local_config"
 
 module Danbooru
   class Application < Rails::Application
+    # Use the responders controller from the responders gem
+    config.app_generators.scaffold_controller :responders_controller
+
     # Initialize configuration defaults for originally generated Rails version.
     config.load_defaults 5.2
     config.active_record.schema_format = :sql