Migrate from SQL templates to SQLAlchemy

[PaulCCCCCCH]

Background: we execute raw SQL statements and SQL templates when creating the database or performing operations. These are defined in back-end/utils/db.py and back-end/utils/db_ops.py.

This makes our database workflow overly complex. It's better to use an ORM library instead. SQLAlchemy is one of the most widely used ORM library for Python and it integrates the best with Flask.

The task is to rewrite the definition of all data tables with Python class and replace our own wrapper functions with SQLAlchemy operations.