backend(api): summary of security measures

[EwoutV]

Actions

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1. Update actions for users (Teacher, Student, Admin) with the correct possible actions #88
   backend(api) +1
   
2. Check other ViewSets for the same issue: remove actions that should not be possible, e.g. deleting submissions

Permissions

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1. Permissions for admin actions in AdminViewSet
2. Permissions for student actions in StudentViewSet
3. Permissions for assistant actions in AssistantViewSet #90
   +0
   
4. Permissions for teacher actions in TeacherViewSet
5. Permissions for checks
6. backend(api): permissions faculties #102
   backend(api) +1
   
7. Permissions for submissions