Add proper authorization to delete assignments and view assignments parts

[zhcet19]

1. Both teachers and students should view the uploaded assignments of teachers.
2. Deleting uploaded assignment should be authority of teachers only (the one who has created )
3. Deleting and editing access of submitted assignments should be given to student that had made that particular submission
4. PDF download access should be there for both teachers and students.

[zhcet19]

@Zaidtech have a look at the aforementioned points and suggest your views

[Zaidtech]

@zhcet19 Sure! Let me handle this!!