Assign imported contributions to the ImportUser

What does this MR do and why?

Assign all contributions to the ImportUser when the destination is a personal namespace.

References

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• Backend part of #472552 (closed)

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Before	After

How to set up and validate locally

1. Open the rails console (gdk rails c) and ensure you have :github_user_mapping and :importer_user_mapping feature flags enabled
2. Migrate a project via GitHub Import and select a personal namespace as destination
3. All imported contributions should be assigned to the ImportUser bot.

FYI, the change also affects Gitea and BitBucketServer when the feature flags gitea_user_mapping and bitbucket_server_user_mapping.