Consider allowing metadata to reuse the previous source value if it was not provided with the request

[Greenheart]

For example, this would simplify editing or verification of data. If the source didn't change, there would one less field for the editor to fill in. And this is especially noticeable when manually editing via a REST client.

However, we would have to add validation if there is no previous metadata for that data point from where we could reuse the source. Or, maybe that would be OK if we just left the source as an empty string? In that case, this could be a good improvement.