Display a QR code on payment request modal

[yate]

part of #42

[yate]

Replacing lightning:// with lightning: in the payment request allowed me to scan it with eclair mobile wallet. Should the real fix be encoding the payment request without the //? https://github.com/lightningnetwork/lightning-rfc/blob/master/11-payment-encoding.md#encoding-overview

[bryanvu]

Thanks for this @yate! FYI I'm going to add a quick followup commit to this to replace "lightning://" with "lightning:" in a simpler way as proposed by @Roasbeef for PR #113.