add uniswap url parameter & add uniswap for arbitrum#200

rahul-kothari wants to merge 3 commits intoefdevcon:masterefdevcon/pretix-eth-payment-plugin:masterfrom

rahul-kothari:uniswap-chain-namesrahul-kothari/pretix-eth-payment-plugin:uniswap-chain-namesCopy head branch name to clipboard