Don't wait for grades if all participants are external

[janno42]

Currently EvaP holds back evaluation results until the grading process is finished (grade document has been uploaded or it has been confirmed that grading is finished but no grade document will be uploaded for the course), if wait_for_grade_upload_before_publishing is True.
In evaluations where all participants are external, we don't want to wait for the process to be finished (this can happen in D-School courses for example). So in this case, the results can be published as soon as the TextAnswer reviews are finished.

(grading_process_is_finished should always be True for Evaluations where all participants are external.)