Stop injecting "fake" DOIs into draft dandisets

Our current publish/DOI creation workflow needs improvement. Currently, when publishing a dandiset, we inject a "fake" DOI into the dandiset metadata in order to allow it to be validated against the `PublishedDandiset` schema; this is because we cannot validate a dandiset without putting a valid DOI in its metadata, but we don't want to hit the DOI server for a real DOI without first validating our dandiset.

Instead of using a "fake" DOI , we can create a "draft DOI" for all draft dandisets. Then, we can promote those to a "Findable" DOI when a publish actually happens.