helper.py: Add method unauthorised_warning to update footer

This method acts as an sanitary check when a normal user is sending
message to a stream where only organization admins are allowed to
send. The method also updates the footer with appropriate warning.

Author: imnitishng

zulipterminal/helper.py

@@ -519,6 +519,43 @@ def match_topics(topic_names: List[str], search_text: str) -> List[str]:
 DataT = TypeVar('DataT')
 
 
+def unauthorised_warning(model: Any, stream_identifier: Any):
+    """
+    Identify target stream based on stream id or stream name
+    and update footer based on the user rights.
+    """
+    stream_id = None
+    if isinstance(stream_identifier, int):
+        stream_id = stream_identifier
+    elif isinstance(stream_identifier, str):
+        for s_id, stream in model.stream_dict.items():
+            if stream['name'] == stream_identifier:
+                stream_id = s_id
+                break
+
+    if not stream_id:
+        msg_footer = (
+            model.controller.view.set_footer_text(
+                "Specified stream does not exist.",
+                5))
+        return True
+    else:
+        if (model.stream_dict[stream_id].
+                get('is_announcement_only', None) == 1
+                or (model.initial_data.get('is_admin', None)
+                    or model.initial_data.get('is_owner', None)
+                    and model.stream_dict[stream_id].
+                    get('stream_post_policy', None) == 2)):
+            msg_footer = (
+                model.controller.view.set_footer_text(
+                    "You are not authorised to send messages "
+                    "to this stream.",
+                    5))
+            return True
+        else:
+            return False
+
+
 def match_stream(data: List[Tuple[DataT, str]], search_text: str,
                  pinned_streams: List[StreamData]
                  ) -> Tuple[List[DataT], List[str]]: