Support for class-level validation

This allows for

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

and similar code without an enclosing ->with().
Update ModelToIdTransformer.php

This is to fix problems with entities with 0 as id, empty will return true if u give 0 as parameter
adding unit test for ModelToIdTransformer in the cases where it gets 0 as id
removed test class from the file
Implemented getAdminsByGroup() / hasGroup() functions
Test if identifier is null or not in twig templates, to determine if we have to generate an edit/create form

Fix Issue #1466 create/update routing problem in form creation, when id is 0
empty_value translation
Fix #1446
Merge pull request #1581 from tiagojsag/empty_value_translation

empty_value translation
Merge pull request #1467 from tashanta/master

Fix form creation url when object id is 0 (Issue #1466)
Merge pull request #1465 from joostfarla/getAdminsByGroup

Simplify retrieval of admin groups
Merge pull request #1453 from javiern/patch-1

Update ModelToIdTransformer.php
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.