Create empty state for offline export history page empty state

What does this MR do and why?

Incrementally builds out the import/offline/export/history page with the GlEmptyState component by default.

Subsequent work will conditionally render this empty state or a list of exports.

References

#587078 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable feature flags:

   offline_transfer_imports

   offline_transfer_exports

   offline_transfer_ui

2. In the app, toggle checkboxes for offline transfer in /admin/application_settings/general#js-import-export-settings

   1. Allow exporting GitLab groups and projects by offline transfer
   2. Allow importing GitLab groups and projects by offline transfer

3. Navigate to /import/offline/export/history to see the empty state

MR acceptance checklist

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