Restart DB migrations with Artifact and File as abstract classes

There are problems with trying to use Artifact and File in optional ontology apps as the top level migrations become dependencies of the (optional) ontology app migrations.

• Remove the existing migrations
• Move Artifact and File to abstract models
• Migrate the current unit tests to create temporary non-abstract classes only within the unit tests
• Roll all existing migrations (up to & including 0005 worker from #13 (closed) ) into a replacement 0001_initial.py
• Provide instructions on how to drop the current DB, recreate it and apply the new 0001_initial.py

Note that this will delay other changes that would require DB migrations