migrate admin permission checks

[PascalinDe]

related to #172

Currently whether someone is administrator or not is decided based on AuthContext which lists the permissions of the given user (a specific combination of permissions indicate an admin user).

Instead whether someone is an admin should directly be decided based on the list of admins managed on the blockchain: https://github.com/c4dt/d-voting/blob/student24spring_access_control/docs/api.md#a3-get-the-adminlist