Move ContactPerson extraction into `dandischema`

[The logic for extracting a dandiset's contact person from its version metadata](https://github.com/dandi/dandi-api/blob/master/dandiapi/api/views/serializers.py#L7-L17) should be moved to dandischema and exported as a function that dandi-api can use.

This will prevent the Contact Person display in the UI from breaking again in the event of a schema change.

Forgot to file this issue and was reminded when I saw https://github.com/dandi/dandi-api/issues/352.

Start by reading the extraction logic in dandi-api/dandiapi/api/views/serializers.py lines 7-17 and review the related context in issue #352. Trace how dandischema is exported and consumed by dandi-api; done means the extraction is provided there as an exported function and the API uses it for the Contact Person display.

Written by the indexing model from the issue text.