* 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 #4419

* 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

* Add `Presenters` group to SimpleCov configuration

* Move validators to app/validators, add to simplecov config

application website validation, don't link to app website if website isn't set,
also comment out animated boost icon from #464 until it's consistent with non-animated version