⚡ Allow to generate a QR Code with lnUrl and get lnUrl address through Phoenix Server

[Tirodem]

Same as #1458 in philosophy
Note that, following here :
https://phoenix.acinq.co/server/api#get-lightning-address
so as long as the first channel is not opened, address won't be available (first channel is opened when balance > initial fees)
so we need to cover these cases :

On CMS Page :

• if I use [QRCode=lnurl] but no lightning address exists, display nothing
• if I use [QRCode=lnurl] and lightning address exists, display qr code with lightning:{lightning_url}

On /admin/phoenixd, when phoenixd is up :

• as long as no channel is created, don't show CTA "Get lightning address"
• if a channel is created, show CTA "Get lightning address"
• when CTA is used, display lightning address

Lightning address, like BOLT12, is not supposed to be changed once Phoenix Server instance is up, then the channel is created.
So there's not special need to call phoenixd to get info to display on QR Code, or each time we go to phoenixd.
We can probably stock bolt12 and lightning address somewhere on database once they're generated to avoid dummy calls to phoenixd API.