API for inactive groups and projects in a group

Problem to solve

On the group overview page, inactive items, such as groups and projects that are archived or pending deletion are not consistently collected in one place (see #442906 (closed)), making it difficult for users to find items that they would like to recover.

We should ensure that all archived items and those that are pending deletion are displayed in the Inactive tab.

Background

For #442906 (closed)

On the group overview we have an Inactive tab that currently just shows archived projects. We want this tab to show archived projects, projects pending deletion, and groups pending deletion. In order to do this we need a new API.

Proposal

• API should return data in a similar format to /groups/<group_path>/-/children.json
• The tree should start at the highest subgroup pending deletion. For example in Top-level group > Foo group > Bar group > Project if Bar group is pending deletion the API would return Bar group > Project.
• All roles should be able to see groups and projects pending deletion in the Inactive tab. Only group Owners and project Owners and Maintainers should be able to action further on them and delete them completely.
• We also need a way to exclude groups and projects pending deletion from /groups/<group_path>/-/children.json. Maybe a new parameter to this API would make sense?