Add krist.club redirect button

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Similar to how clicking the dynmap opens a new tab to the location, it would be nice to be able to click on a button that redirects to the krist.club transaction submission page when having an item pulled up (https://krist.club/send with the necessary query parameters).

The details could all be filled in to purchase 1 item, or an amount selector could be added to the item within shopindex itself. The prior issue about /pay could also benefit from being able to adjust the amount of the selected item.