Create scaffold for offline import form

What does this MR do and why?

Starts the build out of the offline/import stepper form.

After exporting groups to AWS from the /offline/export/ page (and the export is confirmed via email notification), a user will navigate to the the /offline/import/ page for the second half of the process.

The MR adds the two tabs (Configure and Import) with placeholder content. Subsequent MRs under #587079 will build out the actual tabs and update temporary tests.

References

#587079

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. Go to /import/offline/import

4. Verify the first tab, click the checkbox and then you can click Continue for the next tab. Click that checkbox and Start import.

MR acceptance checklist

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