Improve "Grade submitted" message

[MrSerth]

When a user submits an exercise to an LTI consumer, he is automatically redirected to one of three possible pages within CodeOcean. Only one of these displays the grade permanently with a nice progress bar. The other two (feedback request and open RfCs) only display a self-closing flash message for about 10 seconds. This is not ideal and many users raised concerns if their grade was submitted. We should change that experience.

[MrSerth]

We've changed how submitting an exercise works. By now, a flash message is displayed once a full score is reached, inviting a learner to "finalise" the exercise. Then, the existing behavior is triggered. Still, some learners don't know how to proceed after replying to an RfC or taking another action as suggested on the following page.