feat(user management): include inactive users in the list of users

Previously inactive users did not appear on the list of users in the user management.
If we would **NOT** change this behavior, it would result in the following consequences:
- If a user would be set to be inactive, they would disappear from the list of users which would be unexpected.
- It wouldn't be possible to change back an inactive user to be active again.
- An administrative user would not be able to tell whether a user is inactive or has never registered on the portal
  without looking at `/backend/admin`.

Closes #268