Add migrated version metadata endpoint

Implement https://github.com/dandi/dandi-api/blob/master/doc/design/draft-metadata-migration.md

Add an endpoint `GET api/dandisets/{dandiset_pk}/versions/{version}/migrated/` that attempts to migrate the version metadata to the current schemaVersion. If the metadata is already at the current schemaVersion, nothing is done. If the migration fails for whatever reason, simply return the current metadata.

The meditor will use this new endpoint to get the metadata for a version, ensuring that whenever a user saves the meditor content, the schemaVersion is as up to date as possible.