Use new /migrated endpoint in the meditor

[dchiquito]

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

The meditor should use the new GET api/dandisets/{dandiset_pk}/versions/{version}/migrated/ endpoint to ensure that saving the meditor always migrates the metadata.

Currently blocked on #565 to add the endpoint

[jjnesbitt]

Closing in favor of #565