Added unique constraint on work's references.

A work can only have one reference for a single type, chapter, and rank.
Updated result class with new unique constraint.
Use work_reference_id, not guessed_referenced_work_id, in unique constraint.

The referenced_work_guesses table's unique constraint was incorrect.  It was
acting as if a persona could only guess a work was the one being referenced
once.  Instead, it should only be providing one guess per work reference.
There is not problem if a persona thinks a work is being referenced by
multiple references.
Updated WorkReference result class with new unique constraint.

The new unique constraint has a new name, since it involves different columns,
but effectively is just switching one column for another in the constraint.