Add Vue 3 compat flag for offline transfer

What does this MR do and why?

Adds the feature flag vue3_migrate_import_offline_transfer . Verifies vue components load correctly.

The related entrypoints:

• pages/import/offline/export/history/index.js
• pages/import/offline/import/index.js
• pages/import/offline/index.js
• pages/import/offline/export/index.js

References

#613597

Screenshots or screen recordings

What	Before and After (unchanged)
Offline Landing page /import/offline
Offline exports page import/offline/export
Offline exports history page import/offline/export/history
Offline Imports page import/offline/import

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. OPTIONAL: Sanity check the pages in the table above starting from /import/offline.

MR acceptance checklist

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