API Fork: Create missing group structure on fork

The fork operation accepts a group id or a namespace.

For a given namespace I would wish the missing groups would be created.

For example:

• Existing structure: group/subgroup
• Non existing group: newgroup

/api/v4/projects/1234567/fork?namespace=group%2Fsubgroup%2Fnewgroup

This should fork the project 1234567 into group/subgroup/newgroup where the subgroup newgroup is also added.

If newgroup already existed this should work as always.