* 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
Since Rails 5.1 missing migration version results in following error:
```
StandardError: Directly inheriting from ActiveRecord::Migration is not supported. Please specify the Rails release the migration was written for:
```
This PR fixes all migration files.
