Maintain the old Group Import/Export as a workaround for migrations over air-gapped networks

Problem to Solve

GitLab migration replaces the old import / export migration, however some users with offline networks can't use the Group Migration because they don't have network connectivity between GitLab instances. They can only work with the export / import way. Also, the Infrastructure team relies on export/import for data recovery.

Proposed Solution

Hide the Import / export functionality behind a Feature Flag, so Self-managed customers who needs to use it will enable it, and will have both the UI and API functionality.