Upgrades and bugfixes to the stake app and shop

Several upgrades and bugfixes to the stake app!

WATT Multi-send added to the stake app, stake.mining.game/multi-send
You can now easily send WATT to multiple recipients, saving gas!

Inventory now shows Buy and Mint buttons for NFTs you don’t have or if you are not logged in.

Also, you can now find all payment options for a NFT in the shop, just add its ID to the url, like shop.mining.game/3 for the XL1 NFT!