Normalization of roll number done

[TirthGada]

normalised the roll number by creating a function named normalize_roll_number which makes the roll number to uppercase

[Prater-12]

Since _create_user is called even on the admin backend for user creation, this approach is a bit more restrictive than adding a similar check to the serializer (an approach that has been proposed in a previous PR), thus only adding this restriction to users. However, I could see escaping certain specific user names like admin or root.