SSE endpoint for paid invoices.

also move very essential stuff from core/tasks.py to tasks.py so things
are more organized.

lnbits/extensions/lnurlp/__init__.py

@@ -9,6 +9,6 @@ from .views import *  # noqa
 from .lnurl import *  # noqa
 from .tasks import on_invoice_paid
 
-from lnbits.core.tasks import register_invoice_listener
+from lnbits.tasks import register_invoice_listener
 
 register_invoice_listener("lnurlp", on_invoice_paid)