WIP: Feature/create lnurl withdraw link

[kiwiidb]

Once merged, resolves https://github.com/BlueWallet/LndHub/issues/119 on the server-side.

• Introduces a new object Widr (Withdrawal) that can be created by an authenticated user and creates an LNURL-withdraw voucher. Expiry is set to 7 days.
• Adds a front-end page that contains the LNURL qr code and lightning: link.
• Adds 2 routes for the LNURL withdraw protocol (WIP: just pseudocode for now).
• Adds the bech32 dependency for the LNURL encoding.

Pseudocode for the WIP stuff:

router.get(withdrawPrimaryAPIRoute + ':secret', async function (req, res) {
    //look up wd from db
    //check status
    //respond with lnurl payload (description invoice must contain secret)
});

router.get(withdrawSecondaryAPIRoute + ':secret', async function (req, res) {
    //look up wd from db
    //check amount
    //check status
    //check if invoice description matches (contains secret)
    //set status to pending in db
    //get token for user
    //use frisbee to call our own server impersonating as user and attempt to pay invoice
    //callback: set to success (== remove) or failed
    //Q: what if payment stuck?
    //respond with lnurl payload and response from frisbee (success / fail / pending)
});

Open question: what if the LN payment gets stuck?

To do:

• Check user balance at time of creation
• Actual LNURL implementation
• Tests

Out of scope of this PR:

• cancel a created payment link before it expires
• set the expiry time of a payment link
• view a list of payment links that you created
• send notification to user that created the withdrawal after it's been redeemed.

Screenshots

[godSaysHODL]

this is absolutely beautiful! no reviews yet?

[godSaysHODL]

@Overtorment @ncoelho Did you guys ever check this out? If so, any thoughts about it? Kiwi is doing some cool stuff here imo