remove dependence on DataDescription.subject_id to support subject-less data

**Is your feature request related to a problem? Please describe.**
We want to support upload of data without a (primary) subject, such as simulation outputs. This is currently supported by the schema but not by the data transfer service, so supporting it here will be very helpful

**Describe the solution you'd like**
Refactor `UploadDataJob.s3_prefix` to pull directly from DataDescription.name rather then calculating the name from the subject ID. In cases where the name is generated from the subject, that will have been taken care of by the data schema and saved in the json, so this will reduce duplicate code and also support cases where the user has overridden the default name.

**Describe alternatives you've considered**

**Additional context**
Add any other context or screenshots about the feature request here.