Implement per-request SQLAlchemy database session

[ProbablyFaiz]

This is made a little bit tricky because we want to support sane database sessions not only within the scope of a Flask request but also when using these library functions generally. So we need some kind of helper method that, depending on whether the session is needed in the context of a Flask request or not, either uses a Flask request variable or some kind of global session.

Info here on Flask request variables: https://stackoverflow.com/questions/60446199/how-to-set-and-access-per-request-variable-in-flask-python