Don't save the session if there are no flash messages to display
Don't save the session if there are no flash messages to display

Indisciminately calling session.save() in flash.pop_messages() was
causing us to save the session on practically every page load,
regardless of whether there were any messages to display.

Somewhat related to veekun/spline-pokedex#116, which was causing a
similar problem.

veekun/spline,veekun/spline