feat(ReceiverVM): add From method to be able to generate from EnvelopeReceiver

EnvelopeGenerator.Web/Views/TestView/DebugEnvelopes.cshtml

@@ -9,7 +9,7 @@
     string encodeEnvelopeKey(Envelope envelope)
     {
         var receiver = envelope.Receivers.First();
-        return Helpers.EncodeEnvelopeReceiverId(envelope.Uuid, receiver.Signature);
+        return Helpers.EncodeEnvelopeReceiverId(envelope.Uuid, receiver.Receiver.Signature);
     }
 
     IEnumerable<IGrouping<EnvelopeStatus, Envelope>> groupEnvelopes(List<Envelope> envelopes)