Updating artifact table schema

  * Making the `repository` column in the `artifact` schema NULLABLE.
  * This provides schema-level support for Option[Project.Repository]` fields
    in `Artifact` types.
  * Should help resolve #869.

modules/infra/src/main/resources/migrations/V14__make_artifact_project_nullable.sql

@@ -0,0 +1 @@
+ALTER TABLE artifacts ALTER COLUMN repository DROP NOT NULL;