Resolve "gitlab.com: Unable to import a project that was just exported"

What does this MR do?

Fixes 2 issues:

1. Due to a DB inconsistency issue, we have NULL IID pipelines (normally in old instances, such as GitLab.com). These pipelines get imported with a NULL IID, it then generates a new one which could clash with future fixed IIDs during the import.
2. We fill in user_id (and other user IDs) after the models are created, this should be done beforehand for pipelines.

This is really a regression from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16221

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #51747 (closed)

Closes #43896

Closes #43840 (closed)