utils: remove Session after we are done using it in set_app_settings

When the Kallithea WSGI application or celeryd is started, TurboGears
app_config.make_base_app calls Kallithea app_cfg.setup_configuration which runs
utils.set_app_settings. That function will read settings from the database and
store them in the global config. It uses a database session which is created on
demand, but this session was not dismissed but left around for the next thing
that asked for a database session. MySQL will by default close connections
after 1 hour, so when celery tried to run a task after 1 hour of inactivity, it
could fail because of the closed connection.

utils.set_app_settings must thus remove the Session after use, just like
auth.set_available_permissions do.

This will thus fix for example some MySQL connection problems seen with Celery.