Buyer payouts as on-the-fly reverse submarine swaps

[Reckless-Satoshi]

This feature will allow buyers to submit an onchain address instead of an invoice. For a fee, they will be able to directly receive their Sats on their cold storage.

Background and motivation in #128 (comment)

TODO:

• Implement LNnode functions to query channel, wallet and total balance.
• DB model for "Liquidity" under the Control app
• Implement LNnode functions to validate an onchain address and suggest fee in sats/vbyte
• Implement utils for submarine swap cost: linearly proportional to available on-chain liquidity.
• New API model for "OnchainPayment"
• New Order props: is_swap (boolean), payout_onchain (foreign key OnchainPayment).
• Logic remake for on-chain address submission (instead of LNinvoice) and payment (instead of routing)
• Add frontend component. Submit a payout invoice as a new tab: "submit onchain address". Only available if enough liquidity is onchain. Must inform user of % fee cost of the swap. Must suggest user a miner fee and ask for input (calculated in frontend assuming ~120 vbytes tx size). Ultimately shows user 2 amounts: exactly how many sats they receive and how many sats it costs.
• Add frontend component. Transaction sent! show TXID. On successful trade window.

[Reckless-Satoshi]

Ready and live in the testnet platform.