Do What Rails Would Have Done for UNIQUE indices

* Use CREATE UNIQUE (against the advice to use ALTER TABLE ... ADD
  CONSTRAINT)
* While this does not create a constraint, relying instead on the
  index for uniqueness, this is the same implementation that a
  constraint would have used and fails in exactly the same way.
  In addition, it's what a Rails migration would do, so it should
  leave us with fewer surprises