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

config/initializers/inflections.rb

@@ -20,6 +20,10 @@ ActiveSupport::Inflector.inflections(:en) do |inflect|
   inflect.acronym 'JsonLd'
   inflect.acronym 'NodeInfo'
   inflect.acronym 'Ed25519'
+  inflect.acronym 'TOC'
+  inflect.acronym 'RSS'
+  inflect.acronym 'REST'
+  inflect.acronym 'URL'
 
   inflect.singular 'data', 'data'
 end