Move artifact schema validation into `submission-worker`

[frawleykid]

Currently, artifact-submit-action does input validation of submitted artifacts. However, it would be best practice for that input validation to live closer to the database, to ensure invalid data does not end up in the database.

If feasible, that input validation should be moved into submission-worker, which can then return the proper error message over its HTTP endpoint in the case of an invalid artifact.

Also see #9.