Use active flag for user deletion

[RHelmert]

Use a active flag for user deletion such that names of deleted users can still be fetched.

• [x] Add isActive flag to user trait

• [x] Change deletion to change the flag (obfuscate info except names for lecturers)

• [x] Change topic in user deletion to be conform to auth and certificate behaviour

• [x] Add query parameter to getAllUsers

• [ ] Change API

• [ ] Update test/validation documentation