[Authorization] User management in web client

[lanterno]

The web frontend has a nice UI for managing users, but most of the functionality is not implemented on the backend

ToDo:

• [ ] List users with their access level (admin, operator) (backend)
• [ ] Add a new user as Admin/Operator (backend)
• [ ] Modify an existing user access level (backend)
• [ ] Delete access for an existing user (backend)
• [ ] Give user permission to vote on a specific election (backend + frontend)
• [ ] Upload a file with list of users who are allowed to vote on a specific election (backend + frontend)

This requires in the beginning a correct setup of roles-to-permissions conversion. It can be handled in the backend in a normal function, or handled in a more sophisticated way using "casbin". This requires a bit of research.

Note: We use the Casbin library to implement Authorization

[PascalinDe]

@lanterno what is the status of this ticket? it seems like parts of it are done?