boxes: Add warning when sending message to announcement streams

Normal users are not allowed to send messages in announcement streams,
currently when an unauthorised user sends message to this stream, nothing happens.
This commit changes this behaviour and adds a footer warning for the same.
In case if the user is composing a new message `c` from another stream the same
warning is displayed or incorrect stream name warning if the stream name does
not exist.

Fixes: #682

Author: imnitishng

zulipterminal/helper.py

@@ -589,3 +589,37 @@ def notify(title: str, html_text: str) -> str:
             # This likely means the notification command could not be found
             return command_list[0]
     return ""
+
+
+def unauthorised_warning(model: Any, stream_identifier: Any):
+    # Identify target stream based on stream id or stream name
+    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

zulipterminal/ui_tools/boxes.py

@@ -14,6 +14,7 @@
 from zulipterminal.config.keys import is_command_key, keys_for_command
 from zulipterminal.helper import (
     Message, match_emoji, match_group, match_stream, match_user,
+    unauthorised_warning
 )
 from zulipterminal.ui_tools.tables import render_table
 from zulipterminal.urwid_types import urwid_Size
@@ -156,7 +157,10 @@ def keypress(self, size: urwid_Size, key: str) -> Optional[str]:
                         msg_id=self.msg_edit_id,
                     )
             else:
-                if not self.to_write_box:
+                if unauthorised_warning(self.model,
+                                        self.stream_write_box.edit_text):
+                    success = None
+                elif not self.to_write_box:
                     success = self.model.send_stream_message(
                         stream=self.stream_write_box.edit_text,
                         topic=self.title_write_box.edit_text,
@@ -751,10 +755,12 @@ def keypress(self, size: urwid_Size, key: str) -> Optional[str]:
                     email=self.recipients_emails
                 )
             elif self.message['type'] == 'stream':
-                self.model.controller.view.write_box.stream_box_view(
-                    caption=self.message['display_recipient'],
-                    title=self.message['subject']
-                )
+                # Check if user is allowed to post
+                if not unauthorised_warning(self.model, self.message.get('stream_id')):
+                    self.model.controller.view.write_box.stream_box_view(
+                        caption=self.message['display_recipient'],
+                        title=self.message['subject']
+                    )
         elif is_command_key('STREAM_MESSAGE', key):
             if self.message['type'] == 'private':
                 self.model.controller.view.write_box.private_box_view(