Add support for drag/drop of workshop activity action events

Setup

1. Log in as admin
2. Create a course
3. Enrol a student in the course (s1)
4. Enrol a teacher in the course (t1)
5. Create a workshop activity
   • Set “Open for submissions from” to day 10 of the current month at 8am
   • Set “Submissions deadline” to day 15 of the current month at 8am
   • Set “Open for assessment from” to day 20 of the current month at 8am
   • Set “Deadline for assessment” to day 25 of the current month at 8am

Calendar as student

1. Log in as s1
2. Navigate to the calendar
3. Confirm that you can’t drag either of the database events

Calendar as t1

1. Log in as t1
2. Navigate to the calendar
3. Confirm that you see the open event for workshop's submission
4. Confirm that you see the deadline event for workshop's submission
5. Confirm that you see the open event for workshop's assessment
6. Confirm that you see the deadline event for workshop's assessment
7. Try the following steps twice. The first time with the open event and the deadline event of submission, and the second time with the open event and the deadline event of assessment.
   1. Try to drag the open event after the deadline event.
   2. Confirm the cell background colour change to RED once you drag the open event on a day after the deadline event including the deadline date itself (don't drop)
   3. Confirm that you get an error message when you try to drop the event on an invalid day (after the deadline event)
   4. Confirm that if you drop the event on a valid day, the calendar is updated correctly
   5. Go to that course you have created, click on Edit settings on the workshop activity.
   6. Confirm that the "open for submission/assessment from" date for the workshop is updated
8. Redo the previous step again twice. But this time move the deadline event. The deadline event should not be able to move before the open event.
9. Confirm that non of the submission events can be moved after either of the assessment events
10. Confirm that non of the assessment events can be moved before either of the submission events