Add action to download model version

Refs https://gitlab.com/teklia/requests/-/issues/810

Now that we have nice models trained on Arkindex instances, ML scientists may want to download them in order to post-process them (evaludation in most case).

The s3_url is already provided by the model version serializer, so it's just a matter of adding a button on the models managemeent interface (right before the delete action)