Added a parent_id to Quiz to track which quiz this one is a clone of

At any given point in time, there would be atmost one 'editable' clone
of a quiz. Questions can be added / dropped from only the clone

Once a worksheet is made for a cloned quiz, the clone ceases to be editable
Thereafter, one can clone the previously cloned quiz or the parent of
the clone or the parent of the parent ... and so on
Merge branch 'dev'