model: Add toggle_topic_resolved_status function to (un)resolve topics.

Fixes zulip#1075
The function calls get_latest_message_in_topic to fetch recent message
in topic to be (un)resolved. It verifies user and editing conditions
using can_user_edit_topic function and finally add or remove
RESOLVED_TOPIC_PREFIX from topic name.

zulipterminal/model.py

@@ -30,6 +30,7 @@
 
 from zulipterminal import unicode_emojis
 from zulipterminal.api_types import (
+    RESOLVED_TOPIC_PREFIX,
     Composition,
     EditPropagateMode,
     Event,
@@ -629,6 +630,31 @@ def can_user_edit_topic(self) -> bool:
         self.controller.report_error("User not found")
         return False
 
+    def toggle_topic_resolved_status(self, stream_id: int, topic_name: str) -> None:
+        if self.can_user_edit_topic():
+            response = self.get_latest_message_in_topic(stream_id, topic_name)
+            if response:
+                time_since_msg_sent = time.time() - response[0]["timestamp"]
+                edit_time_limit = self.initial_data[
+                    "realm_community_topic_editing_limit_seconds"
+                ]
+                # Don't allow editing topic if time-limit exceeded.
+                if time_since_msg_sent >= edit_time_limit:
+                    self.controller.report_error(
+                        " Time limit for editing topic has been exceeded."
+                    )
+                else:
+                    if topic_name.startswith(RESOLVED_TOPIC_PREFIX):
+                        topic_name = topic_name[2:]
+                    else:
+                        topic_name = RESOLVED_TOPIC_PREFIX + topic_name
+                    args = dict(
+                        message_id=response[0]["id"],
+                        topic=topic_name,
+                        propagate_mode="change_all",
+                    )
+                    self.update_stream_message(**args)
+
     def update_stream_message(
         self,
         topic: str,