Ensure that SideShift transactions are detected correctly as 'success' on the client

[Klakurka]

This is actually trickier than before as we now rely on OP_RETURN which isn't supported by SideShift.

We could possibly add some extra handling on SideShift txs but it would require using their deeper API integration (instead of the simple pop-up one). However we do this, we'll need to client to also recognize that the transaction was via SideShift for a particular amount and that it's somehow associated with them (eg. we can pass them back the SideShift invoice ID if it knows that) since we won't be able to rely on the payment ID in OP_RETURN.

This feels very "custom BIP70"-y to me but might be unavoidable. Lots of opportunity to be clever here with how we approach it.

Related to #187.