Add REST API via Django REST Framework

[mruffalo]

Now that we're reasonably happy with the data model, add a REST API to this application, with possibly-autogenerated serialization of the data models present in the application.

Work with the Harvard group to ensure this API meets their needs, especially in terms of how much information is returned via a single API call vs. how many API calls are necessary to get the relevant information for each tissue.

[mruffalo]

Endpoint that I'm reasonably sure will be necessary: "get metadata for the most recent data products of a particular tissue, newest for each assay, with download links for large data files and one or more S3 URLs to drive a Vitessce view"

[bherr2]

The MC-IU team would also be interested in this API