Namespace leftovers

[ciyer]

The namespace build concentrated on the core functionality, but there are some loose ends that should be tied up.

• Folder rename The spec and and implementation for working with namespaces is in older components/renku_data_services/group/. This should be renamed to components/renku_data_services/namespace/
• Pagination field unification In the query parameters the field is called per_page, but in the response it is per-page. We should pick one canonical name and use it everywhere (also impacts other V2 endpoints with pagination, like projects)
• Find projects by namespace/slug Endpoints that operate on one project currently take the project ID to identify the project. They should be changed to alternatively accept the namespace and slug to identify the project.
• Delete group also deletes any projects in that group Is this the desired behavior?