fix deprecations: use choices_as_values=true and flip content of the choice options

Form/CustomFieldsGroupType.php

@@ -47,7 +47,8 @@ class CustomFieldsGroupType extends AbstractType
         $builder
             ->add('name', 'translatable_string')
             ->add('entity', ChoiceType::class, array(
-                'choices' => $entities
+                'choices' => array_combine(array_values($entities),array_keys($entities)),
+                'choices_as_values' => true
             ))
         ;