Reset puzzle modal form upon successful submission.

[jpd236]

See #2274

[jpd236]

This causes a regression that can lead to errors when saving existing puzzles. Simple repro steps:

• From the puzzle list page, click the edit button on a puzzle.
• Save without making any changes
• Repeat above steps again

I think the problem is that upon successful save, the form's title state is set to "" (and titleDirty set to false) in the onSubmit callback. So if we reload the dialog for the same puzzle and try to save again, the title doesn't get reset.

Need to find a better way to clear state here.

[jpd236]

I think this is now fixed. PuzzleModalForm now exposes a reset() function to reset the form state. PuzzleListPage calls reset upon successful puzzle creation. This leaves the behavior unchanged for editing existing puzzles, but keeps the clearing for new puzzles.