Store artifact's storage type (local vs. remote) in `artifacts` table

[neutralino1]

Full artifact serializations can be stored on the user's machine or in a cloud storage location (e.g. S3). It would be practical to store this information in the artifacts table.

[augray]

removed "good first issue" just because I figure anything involving a DB migration might not be ideal for somebody's first issue! We can add it back if you disagree.