Scenario:

• A entry is purged from cu_changes after 3 months
• A user calls the REST API to add client hints data for this purged entry
• The entry is then saved but never purged, as the purging script won't find it

In this situation a user could have client hints data kept for over 3 months, which would be a major issue.

Thanks to @dom_walden for spotting this.

Acceptance criteria

• Prevent insertion of mapping rows if the reference ID with associated map ID does not refer to an existing row
• Test that this behaviour occurs