Assign issues / MRs created by unknown users during project import to the ghost user, rather than the project owner

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e.g., this line in GitHub import:

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/github/import.rb#L251

This has come up in at least one support request, where the project owner did not care to suddenly become the author of all these issues.

The ghost user was added after imports, and I think it makes a lot of sense to start using it here instead of project owner.

/cc @mydigitalself