Model validations: Validate the nature of the parent for `Group` & `Namespace`

Based on the discussion from !52658 (comment 501222860)

There appears to be a roadblock once I changed the code to parent.execute_hooks and specs complain of undefined method execute_hooks for #<Namespace:0x000055d2f4498fc8> for some specs. (Failing test, Another failing test)

This is because these tests are creating subgroups under Namespace (and not Groups). AFAIK, namespaces cannot/would not have subgroups under it. So building such a structure in specs appears to be wrong. Shall I fix these specs?

Maybe we should also raise "cannot create a group under a namespace" unless group.parent.group? in group factory to avoid these cases. WDYT?