Forbid assigning license for EmbargoedAccess

Seeing a very permissive licenses alongside with ` Access Information: dandi:EmbargoedAccess` on https://gui-staging.dandiarchive.org/#/dandiset/101126/draft made it feel wrong: I can download, forget that it is embargoed, look into metadata for license field, see that is nice free and open and then go sell it for a 1 million $.
I think we should better disallow setting (especially open) license to embargoed dandisets, and setting a license should be a part of the "Unembargo" process: if you assign open license, means dandiset is no longer restricted in its redistribution.