Ability for organizers to delete submissions 

Resolves issue #542 

## Tasks

- [x] Delete button
- [x] Confirm dialog
- [ ] Ask Francis -- does this make sense? does it work well (del button not TOO close to other stuff)

## Tests

- [x] Delete view 404 if not owner of competition submission belongs to
- [x] Delete view 200 if owner
- [x] Delete view 302 if not logged in