refine "save" message capa

Per @jinpa:
> Looking again now at the wording in 6eb78cc I'm worried that "saved
> by not graded" doesn't make crystal clear that they haven't been
> _submitted_.  Given that we're not auto-submitting at the deadline
> (someday let's add that feature), can we change the wording to:
> "Your answers have been saved but not submitted. Click 'Submit'
> when you're ready."