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/validators/url_validator_spec.rb

@@ -2,7 +2,7 @@
 
 require 'rails_helper'
 
-RSpec.describe UrlValidator, type: :validator do
+RSpec.describe URLValidator, type: :validator do
   describe '#validate_each' do
     before do
       allow(validator).to receive(:compliant?).with(value) { compliant }