Let the user add their own puzzles

[catchouli]

Occasionally I see a tactic in a game or a puzzle on another site and want to add it to spaced repetition so I don't forget it. Like Anki, it should be possible to add your own content. It would involve a two step process: providing a board editor to get the intial FEN of the puzzle, and then allowing the user to input the desired moves. It would also need to be possible to edit or remove puzzles, as long as they aren't the built in ones, and puzzles added by a user would likely need to be 'owned' by them. I think the rating fields on puzzles should be changed so they're not NOT NULL anymore as it doesn't really make sense for them to affect the ratings, and we can represent them using an Option instead. A card would automatically be created for any new puzzles added, and we'd need foreign key constraints too to make sure that when a puzzle is removed, the associated cards are cleaned up too and don't show up broken or blocked in the review queue.