Constraint validators should subclass ConstraintValidator instead of implementing ConstraintValidatorInterface directly

Symfony provides Symfony\Component\Validator\ConstraintValidator as a base class for constraint validators. This class helps to provide interoperability between Symfony 2.8 and 3.0, so should always be subclassed rather than implementing ConstraintValidatorInterface directly.

Also note that by subclassing ConstraintValidatorInterface, any class that subclasses Constraint and also implements ConstraintValidatorInterface must be split into two classes for the Constraint and the validator.

Core classes were breaking both of these patterns, but will be updated for the 8.3.0 release in preparation for updating to Symfony 3.