Put safeguards on refresh_metadata script

[dchiquito]

https://github.com/dandi/dandi-api/blob/master/scripts/refresh_metadata.py

This should not impact published things.

[dchiquito]

An asset can be part of a draft version and a published version. It shouldn't be refreshed if it is part of a published version.

[waxlamp]

@dchiquito, can this be closed?

[dchiquito]

Versions are protected adequately, but the assets safeguard still needs to be added.