Aesthetic improvement and disabling of rotating start/goal

The grid is prettier.
You can't rotate the computer and goal.
I think we should do an early return if the cell is goal or start. With this code, we still call `setGridViewToMatchModel` even when nothing changed, which seems wasteful. I think the best course of action would be to just add the following code before the call to `[self.gridModel rotateClockwiseCellAtRow:row col:col]`:
```
if ([self.gridModel isGoalAtRow:row col:col] || [self.gridModel isStartAtRow:row col:col]) {
  return;
}
```