Utility function to get central id from username

Fixes TODO from gerrit 88251, SpecialMWOAuthListConsumers.php, so that
Special:MWOAuthListConsumers can be run on any wiki instead of the
central wiki only.

Change-Id: Ie17cacd68452ffea808d63b3d5b74078577c75be
Merge "Utility function to get central id from username"