* 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
* Fix#2119 - Whenever about to send a HTTP request, normalize the URI
* Add test for IDN request in FetchLinkCardService
* Perform IDN normalization on domains before they are stored in the DB
