Prepare Mastodon for zeitwerk autoloader (#15917)

* Prepare Mastodon for zeitwerk autoloader (Rails 6)

Add inflections and rename/move a few classes.

In particular, app/lib/exceptions.rb and app/lib/sanitize_config.rb
were manually loaded while still in autoload paths.

* Add inflection for Url → URL

spec/lib/sanitize_config_spec.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'rails_helper'
-require Rails.root.join('app', 'lib', 'sanitize_config.rb')
 
 describe Sanitize::Config do
   describe '::MASTODON_STRICT' do