Change time when signature is checked

[ligi]

Currently we check the signature when submitting the transaction

https://github.com/esPass/pretix-eth-payment-plugin/blob/inlining-checkout/pretix_eth/views.py#L81

but it should be checked before even crafting the transaction.
This is especially important when allowing WalletConnect / other providers in single receiver mode as then the Wallet could be a smart contract wallet and the normal recover has more failure paths.