Resolve issues affecting the bulk editing of new metadata fields

[bjester]

Summary

Description of the change(s) you made

Refactors the helper that defines the getter for nested object fields, used for the new metadata, so that it extracts the object keys during the mapping of nodes, and produces the intersection of those values between all selected nodes

Manual verification steps performed

1. Opened a channel for editing
2. Selected multiple nodes
3. Set metadata fields
4. Finish editing
5. Verified metadata was appropriately saved
6. Open only one of the selected resources for editing
7. Verify the previously set metadata is shown
8. Make further edits to the single node's metadata
9. Verified metadata was appropriately saved

References

Fixes #3663

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Contributor's Checklist

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Reviewer's Checklist

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[rtibbles]

Code changes make sense, and fix works as described.