Update importers to not create placeholder references

Background

A new reassignment process has been introduced that eliminates the need for placeholder references. However, importers are still creating these references as a precautionary measure in case of rollback requirements.

Problem

Now that the new reassignment process has been successfully deployed and proven reliable, continuing to generate placeholder references is unnecessary and creates technical debt.

Proposed Solution

Update importers to stop generating placeholder references for:

• Contributions assigned to placeholder users. For contributions linked to Import User, we should keep creating references.
• Contributions with a database index on the corresponding user_id association.

See proof of concept for reference