[refactor] Groups for access

[Azecko]

Since the app is now called CEREAL and not CREP, the groups used for the access may need to be refactored.

• CREP-access → Having access to the CREP calendar
• CREP-admin → Having access to the CREP calendar & admin features about CREP
• CEREAL-access → Having access to the exams table
• CEREAL-admin → Having access to exams table & /admin page

This may need some more thinking, questions like : In the CREP-admin group, the CEREAL-admin group should directly be included.
People that are in the CREP-admin group but not CEREAL-admin group should still be able to see the /admin page, but just to edit the emails about CREP

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