Forbid negative premium prices

[gbrodman]

This hasn't been an issue, but it seems like a good idea. Note that I'm allowing prices of 0 here just in case there's some shenanigans at some point in time where we want a domain to technically be premium without having a cost.

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[CydeWeys]

I wonder if there's anywhere else in the codebase we should enforce prices/fees being non-negative? Maybe in the Fee classes, and something in the Tld class that sets the standard create/renewal price?

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[CydeWeys]

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[gbrodman]

TLDs are an odd case because those are just loaded automatically from the YAML. For Fees, we do already verify that it's nonnegative.

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[CydeWeys]

But we could throw an error on negative values when loading from the YAML, right? Also the YAML doesn't even have to be checked into source control (for non-production domains).

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[gbrodman]

it's not quite that simple but kinda yeah, doing that in a separate PR

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