User: Allow newSystemUser() to create over anonymous actors

User: Allow newSystemUser() to create over anonymous actors

Various maintenance scripts assume reserved usernames like
"MediaWiki default" exist, but since they're reserved
User::isUsableName() returns false and therefore the actor migration
created them as anonymous actors. Which would then prevent those
maintenance scripts from using User::newSystemUser() to ensure they
actually exist.

This adjusts User::newSystemUser() to be able to create users for
those anonymous actors.

This also adjusts uses of "MediaWiki default" in core to create it as a
system user.

Bug: T236444
Change-Id: I59a646df36ff9343cc43c05aa20b2b69b2ee124a
(cherry picked from commit 685b505628099a027ab5c9451502f522b489c109)