* 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

- Disallow links with relative paths
- Disallow iframes with non-http protocols and relative paths

Close #13037

* Add test

* Fix code for sanitizing nested lists stripping all tags

Fix #10834