model: Handle (un)resolving topics.

Fixes zulip#1075
First using get_messages api method to get most recent
message sent by user in the topic for which TopicInfoView
is toggled.
Then using fetch message to edit the topic and propagate
mode as change_all to (un)resolve topics.

zulipterminal/model.py

@@ -26,6 +26,7 @@
 
 from zulipterminal import unicode_emojis
 from zulipterminal.api_types import (
+    RESOLVED_TOPIC_PREFIX,
     Composition,
     EditPropagateMode,
     Event,
@@ -559,6 +560,38 @@ def update_private_message(self, msg_id: int, content: str) -> bool:
         display_error_if_present(response, self.controller)
         return response["result"] == "success"
 
+    def toggle_topic_resolved_status(self, stream_id: int, topic_name: str) -> bool:
+        request: Dict[str, Any] = {
+            "anchor": "newest",
+            "num_before": 1,
+            "num_after": 0,
+            "narrow": [
+                {"operator": "sender", "operand": self.user_email},
+                {"operator": "stream", "operand": self.stream_dict[stream_id]["name"]},
+                {"operator": "topic", "operand": topic_name},
+            ],
+        }
+        response = self.client.get_messages(request)
+        if response["messages"] != []:
+            topic_msg_id = response["messages"][0]["id"]
+            if topic_name.startswith(RESOLVED_TOPIC_PREFIX):
+                request = {
+                    "message_id": topic_msg_id,
+                    "topic": topic_name[2:],
+                    "propagate_mode": "change_all",
+                }
+            else:
+                request = {
+                    "message_id": topic_msg_id,
+                    "topic": RESOLVED_TOPIC_PREFIX + topic_name,
+                    "propagate_mode": "change_all",
+                }
+            response = self.client.update_message(request)
+            display_error_if_present(response, self.controller)
+        else:
+            self.controller.report_error("You have not sent any message to the topic.")
+        return response["result"] == "success"
+
     def update_stream_message(
         self,
         topic: str,