Ensure SudokuGridViewController correctly updates its puzzle

Make SudokuGridViewController only update a cell if its an answer
cell that has been edited. Otherwise completely replace the cell
and blow away all other cells answers upon editing.
Ensure Sudoku puzzles save and load correctly

Ensure that Sudoku puzzles don't clear givens when
other cells are saved, that they are loaded properly
from json, and that answer cells display their
candidates when showSolution is not enabled.