Hao-Hsiang Song / topics-auction

- Set H2 database to persistence mode (db is saved to target/h2db); data added to db will not disappear after stopping the program
- H2 database can now be viewed at http://localhost:8080/h2-console (JDBC URL:jdbc:h2:file:./target/h2db)
- Add feature adding item to cart; cart, with the same id as user's, automatically create when adding item to cart
- Comment out User and related classes for now