tests: accept py3 output in test_list_valued_setting_update

The old behaviour seems completely broken and useless and thus evidently unused
... but the new result seems slightly better.

codingstyle: trivial whitespace fixes

Reported by flake8.

db: clean up SQLAlchemy session flushes

Many calls to Session().flush() were completely superfluous and have
been removed. (See also the note on "flush" in the contributor docs.)
For the remaining calls, a comment has been added to explain why it's
necessary.

db: remove superfluous Session.add calls

Don't re-add objects to the SQLAlchemy Session just because they were
modified. Session.add is only for freshly constructed objects that
SQLAlchemy doesn't know about yet.

The rules are quite simple:

When creating a database object by calling the constructor directly, it
must explicitly be added to the session.

When creating an object using a factory function (like "create_repo"),
the returned object has already (by convention) been added to the
session, and should not be added again.

When getting an object from the session (via Session.query or any of the
utility functions that look up objects in the database), it's already
added, and should not be added again. SQLAlchemy notices attribute
modifications automatically for all objects it knows about.

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.