Detect RBF-transactions and offer to track them.

fixes #78

frontend/src/app/services/state.service.ts

@@ -24,6 +24,7 @@ export class StateService {
   mempoolStats$ = new ReplaySubject<MemPoolState>(1);
   mempoolBlocks$ = new ReplaySubject<MempoolBlock[]>(1);
   txConfirmed$ = new Subject<Block>();
+  txReplaced$ = new Subject<Transaction>();
   mempoolTransactions$ = new Subject<Transaction>();
   blockTransactions$ = new Subject<Transaction>();