Add flag to RenameuserSQL to skip username check

It would be very helpful to use RenameuserSQL from CentralAuth to
rename CentralAuth users on the local wikis. However, CentralAuth
will rename the user in the local wiki's user tables (all wikis at
once), so it would be great if RenameuserSQL would not fail if that
update has already been made.

Change-Id: I5a8abd6fb2c1abc54c727adaffa4566d4f20617f
Merge "Add flag to RenameuserSQL to skip username check"