Split from T70069.

https://www.mediawiki.org/wiki/Special:Log/Rothbard registered today to use phabricator, but got «To use Connected Apps on this site, you must have an account across all projects. When you have an account on all projects, you can try to connect "phabricator-production" again.».

The drawbacks explained at T70069 are not as important for mediawiki.org, and we can't force non-Wikimedia users of MediaWiki to deal with Wikimedia projects username conflicts just to file a bug. We can at least avoid that new registrations encounter the problem, by making them register a username that can be used in phabricator.