db: always add to session in Setting.create_or_update There's no use case for not adding the newly created Setting to the SQLAlchemy session (and thus, once we commit, the database). With this change, all the various "create" model methods add their result to the session.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.