Only update modified date from the API

Currently, the `modified` field is set to update on save, which means any change to a Dandiset/Version/Asset will update the `modified` field, including updating validation status or any other fields not directly related to content. This means that the modified date gets updated if, for example, an asset is revalidated.

We should only be updating the `modified` field when changes are made to the content of the model, or at least only when changes are made through the REST API.

Related issues (edit this list as more are found):
- https://github.com/dandi/dandi-archive/issues/1227
- https://github.com/dandi/dandi-archive/issues/1432