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

app/validators/url_validator.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-class UrlValidator < ActiveModel::EachValidator
+class URLValidator < ActiveModel::EachValidator
   def validate_each(record, attribute, value)
     record.errors.add(attribute, I18n.t('applications.invalid_url')) unless compliant?(value)
   end