Sync'ing the ACL and Auth groups adds the sentinel user if there's any orphans

[mome-borogove]

Clicking the button on the ACL page that synchronizes that ACL with any corresponding Auth groups will also add Alliance Auth's internal sentinel users (which displays as "deleted") if there's any orphaned characters on the ACL (i.e., characters without a Uni main). This is the culprit: https://github.com/EVE-University/whctools/blob/main/whctools/utils.py#L207 We should be checking the user before blindly adding them.