Move subgroups and projects pending deletion from 'Subgroups and projects' to 'Inactive' in group overview

Problem

Groups pending deletion are currently displayed in the Subgroups and projects tab of the group overview page. Once #436392 (closed) is implemented, we should move groups pending deletion to the Inactive tab instead, so that users can have a focused view on active subgroups and projects only.

Note: Projects pending deletion are already listed in the current Archived tab (soon to be Inactive tab).

Proposal

• Move groups pending deletion to the Inactive tab, so that users can have a focused view on active subgroups and projects only in Subgroups and projects.
  • If the group pending deletion is nested at a lower level than the the first level underneath the current group, move only the group pending deletion (and all its subgroups and projects) to the Inactive tab. This means that if the group structure is Top-level group > Group > Subgroup > Project, and Subgroup is moved to pending deletion, only Subgroup should move to the Inactive tab, but Group remains in Subgroups and projects.
  • Ensure that all subgroups and projects of the group pending deletion are also marked as Pending deletion in the Inactive tab when unfolding the nested structure. See also #457718 (closed).
• Ensure that Archived projects and groups and projects Pending deletion are presented in the Inactive tab only.
• 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.
• Add a new section to the docs that explains how to "View groups pending deletion", similar to View projects pending deletion.

Follow the styling proposal discussed here.

Implementation guide

1. Change app/assets/javascripts/groups/service/inactive_projects_service.js#L18 to use endpoint added in #467265 (closed)
2. Update specs in spec/frontend/groups/service/inactive_projects_service_spec.js