nutshell no longer works with lnbits back-end as of lnbits v1 due to sse having been removed

[tsjk]

nutshell uses sse to stream invoices from lnbits. In lnbits v1 sse has been removed and replaced by a websocket service. nutshell thus needs to be updated to use that when the back-end is an lnbits v1 instance.