Use repayment_amount in extension when signing and repaying loan

[bonomat]

In the extension we always show the principal amount instead of the repayment amount. Unfortunately it does not seem to be possible to get hold of this number from what is available in https://github.com/comit-network/waves/blob/06701f852298e28bbba69d5d83ed44e36301775a/extension/wallet/src/wallet/extract_loan.rs#L11

Originally posted by @bonomat in https://github.com/comit-network/waves/issues/233#issuecomment-898844781