Cleanup DB migrations

[brassy-endomorph]

Followup to #272

• Ensure consistent naming conventions (also noted in SQLAlchemy metadata is not acutally added to the extension #586)
• Ensure all tables have an explicit __tablename__
• Ensure all tables name are plural (for consistency)
• Remove empty revision c2b6eff6e213_.py (supersedes [pedantic] Migration version c2b6eff6e213 should have a short description appended to the name. #547)
• Ensure (if possible) linear migrations (for now) to support easier auto-testing of migrations (relates to Alembic migrations should have associated test cases #555)