Migrate release_date to DateTimeField for consistency with PurlDB model and incoming data

In ScanCode.io and DejaCode the `Package.release_date`fields are defined as `DateField`.
This is not the case for PurlDB, where it is defined as a `DateTimeField`.

The data returned by the PurlDB API under the `"release_date": "2017-11-01T00:00:00Z"` format cannot be set on the ScanCode.io model as-is, it needs to be converted, and it's a source of issues.

We should migrate to a `DateTimeField` for model consistency across the apps.