Pull Request: C4_Husseny_504_Grade_Solution_4.30

Issue: #504 
Reviewers: @rami-khalil @mohamedsaeed93 @Sharaf93 @fadel1493 @mimikian 

For scenario:
Do the same as marking a solution choose DSD assignment 
choose view submissions then mark and grade
then try to add a grade and proceed afterwards
try to add an empty grade
try to add a grade higher than the final grade
try to press done marking without adding a grade