model: Use stream accessor function to access stream name.

This commit replaces the direct use of stream_dict to access the 'name'
property, with the stream accessor function introduced in the earlier
commit.
Since the new stream accessor function accesses subscribed streams, was
subscribed streams and never-subscribed stream, the keyerror issue
described in issue #816 is resolved.

Author: theViz343

zulipterminal/model.py

@@ -890,7 +890,7 @@ def is_muted_topic(self, stream_id: int, topic: str) -> bool:
         """
         Returns True if topic is muted via muted_topics.
         """
-        stream_name = self.stream_dict[stream_id]["name"]
+        stream_name = self.stream_property(stream_id, "name")
         topic_to_search = (stream_name, topic)
         return topic_to_search in self._muted_topics