Description

Following feedback on the original approach (excluding Cancelled from quota),
this PR takes the suggested route: let organizers hard-delete soft-deleted
submissions so they can free up quota manually.

Issues this PR resolves

• Point 2 from More control over submissions for organizers #2249

Changes

• Frontend (submission_manager.tag): Add a delete button (trash icon)
  on soft-deleted submissions, visible to organizers only.
• Backend (models.py): Guard self.data access in delete() and
  soft_delete() — after a soft-delete, self.data is None, which caused
  an AttributeError when trying to hard-delete.

A checklist for hand testing

• As an organizer, soft-delete a submission, then confirm the trash icon appears
• Click the trash icon and confirm the submission is permanently deleted
• Verify quota is freed after hard-delete
• Confirm non-organizers do not see the delete button on soft-deleted submissions

Checklist

• Code review by me
• Hand tested by me