PBS Permissions for creating a PBS only as a child

Description: Need the ability to limit authors from creating PBS at root.

Use Case/Example: My BOE Author may been to create a Level of the PBS, but they should not be able to create a new root level. For example, the proposal may have WBS as the PBS to the 3rd level, and the author does need to create a 4th level, but they should not be able to create a new 3rd level.