[Validator] Extracted message interpolation logic of ConstraintViolation and used the Translation component for that
[Form] Adapted Form component to translator integration in the validator
[TwigBridge] Adapted TwigBridge to translator integration in the validator
[FrameworkBundle] Adapted FrameworkBundle to translator integration in the validator
[DoctrineBridge] Adapted DoctrineBridge to translator integration in the validator
[Form] Added upgrade instructions to the UPGRADE file
[Form][Validator] Added BC breaks in unstable code to the CHANGELOG
[Validator] Changed validator to support pluralized messages by default

This was implemented in order to satisfy Drupal's requirements for a
singular and a plural message whenever a message is passed to their
implementation of transChoice().
[Validator] Fixed failing test