(CE-1000) Assign user page creations to the default welcomer

Prior to the rewrite, the user page creations were assigned to the
default welcome user (User:Wikia) due to the overwrite of $wgUser
global rather than passing it to the doEdit method. This restores the
expected functionality by passing the default welcome user to doEdit
rather than the sender object.
Merge pull request #4510 from Wikia/CE-1000

(CE-1000) Assign user page creations to the default welcomer