Use global session factory [refactor]

[sssoleileraaa]

Description

Instead of passing around sessions and sessionmakers between storage functions, the controller, and our gui, use a global session factory, as describe in Sqlalchemy docs covering Sessions: https://docs.sqlalchemy.org/en/13/orm/session_basics.html

This makes it easy to share a session across separate sections of code if we have to.

[sssoleileraaa]

@rocodes - in addition to #1108, also this on your radar