Add stacking calculator to lockstacks.com

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I see users asking for one in the social channels. Especially new users like to get an idea what the yield will be on their Stack prior to dipping their toes into Stacking STX. Simply great for managing the users expectations.

We could model it after this stacking calculator, which has not yet been updated to PoX-3
https://stackingcalculator.com/
Repo: https://github.com/OmurCataltepe/Stackulator