avoid passing around sdc_home and instead pass a session maker

[heartsucker]

We have these lines in our code 3 times

engine = make_engine(sdc_home)
Session = sessionmaker(bind=engine)
session = Session()

For simplicity, we may want to pass the Session around instead. This will also reduce the need to mock as heavily in tests.

[redshiftzero]

This is done in https://github.com/freedomofpress/securedrop-client/compare/refactor-session for very closely related ticket #382. Closing this ticket in favor of #382 so that we can track progress in one place