Handle Embargoed Dandiset Creation

[garrettmflynn]

The current PR for Dandiset creation (#522) uses a JavaScript API to handle requests to the DANDI server—but this currently doesn't seem to be able to handle embargoed Dandisets. For requests involving these, I get a 401 (Unauthorized) error.

Ideally, the JavaScript API can modify itself to pass the required authentication information (which appears to be different than the Dandiset creation auth info...) so these Dandisets can be retrieved immediately after creation.

As a fallback, we can instead start making requests to the Python API.