Issue when merging snapshots

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Steps to reproduce

- Use the Zotero connector and save the same website twice including snapshots
- Sort the items list by Date Added (optional)
- Select the earlier snapshot attachment and delete it (Shift-Del).
- Select the two parent items, right-click, select "Merge Items...".
- The merged item doesn't have a snapshot attachment
- It would be expected that the later snapshot isn't removed