Support for class-level validation

This allows for

    $errorElement->addConstraint(new UniqueEntity(['fields' => 'email']));

and similar code without an enclosing ->with().
add placeholder for empty short description fields
Merge pull request #1306 from luzat/classvalidation-master

Support for class-level validation
additional translations
missing translations placeholders
Merge pull request #1437 from tiagojsag/short_object_placeholder

add placeholder for empty short description fields
added model transformer boolean to sonata boolean
Merge #1303 and tweak code to be BC
Add overridable history revision timestamp template.
Update documentation.