Prepare to drop workflow_user_* fields

A followup patch[1] will drop these fields completely.  This patch
makes them nullable so that the followup patch can stop inserting to
them. Once the followup is applied these fields will be removed 
completely. A bug[2] has also been filed to have this change applied
in WMF production.



[1] I7c759b46995c045405426239e95f40147e8aaecb
[2] https://bugzilla.wikimedia.org/show_bug.cgi?id=72842

Change-Id: Ib5d2d101def3c331a3128b80101ee7b2d093aee9
Merge "Prepare to drop workflow_user_* fields"