An alternative way to validate projectId on creating/updating backlogs &
sprints -- Nested json object (the project object) is no longer needed.
The creation and update for project field on backlog/sprint object can
now be validated with different constraint e.g. the projectId must be
set on backlog/sprint creation, but can be null in the patch when
performing update.  This was not easy to implement with @Valid on
project field and the ValidateOnPartial group.