Resolve "Make exam a many to many field on StudentInfo model"

core/migrations/0005_auto_20200707_1456.py

core/migrations/0006_auto_20200810_1115.py

core/migrations/0007_auto_20200922_1026.py

core/migrations/0008_auto_20200922_1148.py

core/migrations/0009_submissiontype_exam_type.py

core/migrations/0010_group_examtype.py

core/migrations/0011_auto_20201103_1211.py

core/migrations/0012_auto_20201103_1228.py

core/migrations/0013_auto_20201103_1248.py

core/migrations/0014_merge_20201123_1252.py

core/migrations/0015_group_exam_type.py

core/migrations/0016_auto_20210902_1140.py

core/models/__init__.py

core/models/group.py

core/models/student_info.py

core/models/submission_type.py

core/serializers/__init__.py

core/serializers/common_serializers.py

core/serializers/group.py

core/serializers/student.py