Remove the "Close Assessment" Button and instead close the assessment after pressing "save" or "discard" assessment

[Luro02]

The current behaviour is confusing, because after you press save, the assessment is not automatically closed. You must press the close assessment button too, which emits this prompt:

that suggests that my previously saved assessment would be discarded.

Instead, I would suggest to have the following three buttons:

1. Submit Assessment
2. Cancel Assessment (will cancel in artemis, discard assessment and close the open assessment)
3. Save Assessment (will save assessment in artemis, then close the open assessment)

[dfuchss]

Save was useful for debugging if it's not closing, but I agree a typical user might not need that