Change lightning: to lnurl: for App URL Schemes

[jackeveritt]

By recommending that lightning: is used to open LNURLs for apps, it causes a clash with apps that don't or never will support LNURL, but use the lightning: scheme.

For example, Bottle Pay probably won't support LNURL (at least initially). And on iOS, if Bottle Pay is the last app that is installed with the lightning: scheme, any web pages or apps on the device will redirect to Bottle Pay by default, with no option to change the default behaviour in the operating system.

On Android the user can change default behaviour, but its buried in the OS settings and is a customer support issue that could be avoided.

Perhaps the scheme should be lnurl: to avoid this.

[hsjoberg]

This is addressed in LUD-17.