Implements a one-step user registration
- Moves old registration controller inside "registration" package
- Creates a new registration controller that creates the User in one step
- Creates an action for email verification
- Add views for the new actions/emails
- Adds methods for new views, in Mailer and TemplatePlugin (without breaking backward compatibilty)
- Adds a state field in Identity trait, to hold the state of email verification
- Adds configuration options to choose the registration method