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."

common/lib/xmodule/xmodule/capa_module.py

@@ -1010,7 +1010,7 @@ class CapaModule(CapaFields, XModule):
         self.system.track_function('save_problem_success', event_info)
         msg = "Your answers have been saved"
         if not self.max_attempts == 0:
-            msg += " but not graded. Hit 'Submit' to grade them."
+            msg += " but not submitted. Hit 'Submit' when you are ready."
         return {'success': True,
                 'msg': msg}