Display script preimage and verification instructions

[Sjors]

A user probably wants to know that the service works as advertised, i.e. that for a given Lightning invoice the bitcoin address will indeed either refund the sender or only be spendable by the recipient using the preimage. Perhaps a downloadable script? Or a Bitcoin Core PR :-)

[alexbosworth]

ACK

Part of this is currently in the codebase and UI but commented out, needs to be made user-friendly