Automatically setup LNBits wallet if none is configured

[ThomasFarstrike]

Currently, to get a working device, the user has to create an LNBits lightning wallet (on their own server, or the "Lightning Piggy demo server":https://demo.lnpiggy.com/ or the "LNBits demo server":demo.lnbits.com or a third party's server). Then they have to enable the right extension(s), configure the extension(s), copy the right API keys and paste them into their device's configuration.

As this is quite a hassle, it would be nice if the device itself does all of these steps, using LNBits API calls, if it notices that none of the relevant settings have been configured.

After creating the wallets and setting itself up, the device should show the "private LNBits URL":https://demo.lnpiggy.com/wallet?wal=abcde981723981723981723981723&usr=abcde981723981723981723981723 (also as a QR code) that contains the wallet ID and user ID, and ask the user to save it, to allow them to access their LNBits wallet from any webbrowser, even if the device has an issue.

To be able to show the private LNBits URL with the wallet and user ID, the device should save them locally into the configuration.

The LNBits API calls that would be required would be:

• create lnbits wallet (resulting in the wallet ID, user ID, Invoice/read key, to be saved locally)
• activate lnurlp "pay links" extension
• call "new pay link" (resulting in the LNURLp, to be saved locally)