Feature/user management

User management to the bare minimum. User list, user add and user delete.

Things missing:

• add user to active group
• check that the user can login
• validate password == repeated_password ¯_(ツ)_/¯
• resolve issue https://gitlab.com/thedrastical/iuvia/management/issues/160
• update user

Related: https://gitlab.com/thedrastical/iuvia/management/issues/163