Calendar modal doesn't show when opened from a modal form

Master only

1. Log in as master
2. Navigate to Reports > Report builder > Custom reports in site administration
3. Create a new report
4. In report editor, press Audience tab
5. Add All users audience type
6. Press Schedule tab
7. Press New schedule
8. Press the calendar icon for the Starting from field
9. Confirm the calendar modal shows above the form
10. Click a different date in the calendar modal
11. Confirm the Starting from field is updated to selected date
12. Save the form (fill in all required fields)
13. From Actions menu select Edit schedule details
14. Confirm the calendar icon still works

Regression test (311/master)

1. Navigate to Dashboard
2. In Calendar block press New event
3. Confirm the calendar icon for the Date field works correctly when clicked
   • It shows above the modal
   • Selecting a date updates the form Date field
4. Navigate to new course creation form
5. Confirm the calendar icon for the Course start date field works correctly when clicked
   • It shows above the form
   • Selecting a date updates the form Course start date field