When running a batch delete as the "Delete page script" user it overwrites the global $wgUser context. This causes all kinds of errors and pollutes user sessions. The changes fixes this to use the built in functionality to specify the deleting user. I did test for any potential security issues regarding this, but I was unable to create a situation in which two users doing the same action would get their sessions taken over. This also fixes the localized deletebatch-system-username not being used when creating the user.
https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/DeleteBatch/+/475839/
See the upper right of the screen shot where the logged in user's session was polluted and overwritten with the "Delete page script" user.
