notif [nfc]: Use TopicName.apiName for encoding NotificationOpenPayload

This isn't part of the Zulip server API -- it's entirely between the
`buildUrl` and `parseUrl` of this class, and we could make up whatever
encoding we like.  But the API name of the topic is the natural thing
to use.

Author: gnprice

lib/notifications/display.dart

@@ -573,7 +573,7 @@ class NotificationOpenPayload {
           TopicNarrow(streamId: var channelId, :var topic) => {
             'narrow_type': 'topic',
             'channel_id': channelId.toString(),
-            'topic': topic,
+            'topic': topic.apiName,
           },
           DmNarrow(:var allRecipientIds) => {
             'narrow_type': 'dm',