model: Add dedicated render method call to presence update handler.

This small commit adds presence renderer function introduced in
previous commit to the handler `start_presence_updates`. This way,
the method `update_msg_box_presence_markers` is called every minute.

Fixes zulip#896.

Author: Ezio-Sarthak

zulipterminal/model.py

@@ -345,6 +345,8 @@ def _start_presence_updates(self) -> None:
                 if hasattr(self.controller, 'view'):
                     self.controller.view.users_view.update_user_list(
                         user_list=self.users)
+                    (self.controller.view.middle_column
+                        .update_msg_box_presence_markers())
             time.sleep(60)
 
     @asynch