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

lib/exceptions.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+module Mastodon
+  class Error < StandardError; end
+  class NotPermittedError < Error; end
+  class ValidationError < Error; end
+  class HostValidationError < ValidationError; end
+  class LengthValidationError < ValidationError; end
+  class DimensionsValidationError < ValidationError; end
+  class StreamValidationError < ValidationError; end
+  class RaceConditionError < Error; end
+  class RateLimitExceededError < Error; end
+
+  class UnexpectedResponseError < Error
+    def initialize(response = nil)
+      if response.respond_to? :uri
+        super("#{response.uri} returned code #{response.code}")
+      else
+        super
+      end
+    end
+  end
+end