Hierarchy widget - Add restrictions for parent type model

should validate that assigned parent is allowed, &7448 (comment 952801954): parent should be of type Issue and child should be of type Task
maximum number of children per parent is 100 (&7448 (comment 961232633))
Issue<->Task relationship should be available in GitLab Free (&7448 (comment 961490289))
should be kept in mind that in future custom definitions should be supported (&7876)

Edited May 30, 2022 by Jan Provaznik