Make renameuser_queue.rq_wiki nullable

This corrects an error in the original DDL for renameuser_queue
introduced in I61acceb. We intended the rq_wiki column to be nullable.
A null value will be used to indicate that the user requesting a rename
is already a global Central Auth account holder.

Change-Id: I342c6dcc19ded36ebab530e98a3d81a7dc188748
Merge "Make renameuser_queue.rq_wiki nullable"