Add double deletion requirement to API

Problem

Delayed deletion will become the default behaviour in %16.0. Users will still be able to immediately delete groups or projects in the UI via a double deletion. However, this is not how the API currently works for projects. For groups (only for subgroups, not for root groups), double deletion is possible via the API, by passing permanently_remove =true and full_path of the subgroup, and we want to bring that functionality to projects API too.

Proposal

The double deletion requirement should be reflected in the API as well.