Workshop activity doesn't recover completed marks for students

Hello,

Workshop activity doesn't recover user completion marks for activity completion if you "unlock completion options" in the activities options. It should recover them like Quiz does.

Steps to reproduce:

1. Configure a workshop with the following activity completion "Show activity as complete when conditions are met" and "Student must receive a grade to complete this activity"
2. Complete the workshop with 2 students that review each other and set workshop phase to closed
3. Check that the students have the completion mark.
4. Open workshop settings. Under "Activity completion" click "Unlock completion options" and save.
5. Check completion report and notice how completion mark have disappeared from the students. 
6. Run the cron and check that the completion mark is still not updated (although the students have been graded).