Omit gl_project_id from Import/Export

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/29813

I don't think changelog is needed here since it's a backstage change, that should have no effect.

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

On hold until https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10853 gets merged.

This MR's milestone has to be the same as https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10853

unmarked as a Work In Progress

merged

mentioned in commit 1a5e84fe