Fixed FK from guessed_referenced_work_id column.

The guessed_referenced_work_id column in th ereferenced_work_guesses table was
referencing the work_references table's primary key.  This is incorrect; it
should reference the works table's primary key.  The type of this column was
also updated from BIGINT to INT to match the works table's primary key type.
Updated result classes to match updated FK.

The reference from the ReferencedWorkGuess result class to the WorkReference
result class was updated to instead go to the Work result class.
Replaced user/algorithm FK with a persona FK.

The referenced_work_guesses table used user_id, algorithm_id, and version to
identify who made a referenced work guess.  This has now been replaced by a
single persona_id column, which references the personas table.
Updated result classes to reference Persona class.

Changed ReferencedWorkGuess to have a relationship with the Persona class,
instead of with the Algorithm and User classes.  Also updated the
ReferencedWorkGuess class with the updated columns and unique constraint.
Advanced version to 0.007.
Added version info for 0.007.