add asset api url to paginated assets view

with the manifest being added (#866), it would be nice if one could hop from that page into an asset without having to copy and paste uuids.

while one can use the uuid to get the assets (programmatically), it would be nice if the dandiset specific asset url was part of the response so that one could interactively navigate in the django interface.

url: "https://{api-server}/api/dandisets/{id}/versions/{version}/assets/{assetid}"

Start by tracing the paginated assets response and the manifest work in issue #866, then identify where asset records are assembled for the Django interface. Done means each asset includes its dandiset-specific URL in the response, using the format given in the issue, so users can navigate to it without copying a UUID.

Written by the indexing model from the issue text.