Reverse Swaps: Allow for custom claim network fee

[kilrau]

As per BoltzExchange/boltz-backend#326 (comment) we want to allow for users to chose a custom sat/vbyte claim network fee via e.g. a slider.

Currently web app estimates fee before calling /createswap in order to be able to give exact receive amounts. The custom fee slider should clearly show how the receive amount changes when bumpgin or lower the fee.