EZP-21035: add BadStateException to ContentTypeService::addFieldDefinition()
EZP-21035: add BadStateException to ContentTypeHandler::addFieldDefinition()
EZP-21035: implement FieldType::isRepeatable()
EZP-21035: fixed checking Content instances for ezuser field
EZP-21035: implement ContentHandler::countInstancesOfContentType() and move logic to the BL
EZP-21035: renamed countInstancesOfContentType() to getContentCount()
EZP-21035: rename isRepeatable() to isSingular()