Add fields for optional donation links/text in app skeleton generator form

[edward-ly]

Title says it all. Any links provided would add a corresponding <donation> tag to appinfo/info.xml. PayPal/Stripe links should also be automatically detected so that the type attribute can be set accordingly.

[provokateurin]

I'm not sure it makes sense to do that. When you are creating a new app you probably don't have ways to donate yet or care about it right away because you're going to want to write some code first before thinking about releasing and donations.

[edward-ly]

That's why I specifically mentioned that the fields be optional. The links don't have to be app-specific either and could rather be e.g. the developer's Liberapay profile, GH sponsor page, etc.

And actually, the app template doesn't necessarily need to have any links included anyway. The generator could just add the necessary attributes and that's it.