Import all subgroups and projects from an exported backup

This relates to #241379 (comment 402977708)

This is the follow up to #241704 (closed)

Goal

1. To create a mechanism to upload a bundle of the group and project exports, and orchestrate the imports of these to the correct locations

The structure of this file will be decided in #241704 (closed)

Why

We need to restore the data from an accidentally deleted group from a back up. There are a lot of subgroups and projects in this group and restoring them manually would take too long.