ATProto (BlueSky) update

@Tom_Sherman asked whether the Discourse username could be made to match the visitor’s ATProto handle.

Well, it turns out that it can! It updates automatically from the preferred_username field of the ID token on every login - and FedIAM can now be configured to put the handle in this field.

It remains to be seen whether mentions will update properly if it changes, though.

@user6 comes from the ATProto world, too, so let’s see what happens if they log in again.