Move creator metadata population to dandischema

As of #351 we are injecting some creator metadata into the dandiset metadata when it is created. Because this metadata may change based on schema version, we should move that function into dandischema.

Start by locating the creator metadata population introduced in #351 and trace how dandiset metadata is created. Then inspect dandischema's schema-version handling; the work is done when this function lives in dandischema and metadata creation uses it without keeping schema-dependent logic in the API server.

Written by the indexing model from the issue text.