extend account creation backfill script to forced account creations by others

Also, do not create accounts with default IP/user agent if we can't
find the relevant info; just skip creating those accounts.

This depends on a CheckUser service defined in commit
I2aba5799b267c93d8975d5e1fd8b6b8fafc4fc08 already merged into
master.

Bug: T378401
Depends-On: I2aba5799b267c93d8975d5e1fd8b6b8fafc4fc08
Change-Id: Ie09ad749e23a342d922367c2ef0d154c0a98e3d4
(cherry picked from commit 9a3ff32fec1c044f0465151575e880c0468e70f1)