Sorting users by registered date assumes users were registered in order

[johnbillion]

On the Network Admin -> Users screen on a Multisite installation, clicking the Registered column header sorts the users by ID. This assumes users were registered in the same order as the order of their sequential IDs.

Importing users breaks this assumption, as a user's registered date may be earlier than their ID otherwise suggests.

Ideally this column header would sort by the user_registered field instead, but this has performance concerns because it's not part of an index.