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.

Fixes: #682

Author: imnitishng

zulipterminal/ui_tools/boxes.py

@@ -751,10 +751,23 @@ 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 (self.model.stream_dict[self.message.get('stream_id')].
+                        get('is_announcement_only', None) == 1
+                    or (self.model.initial_data.get('is_admin', None)
+                    or self.model.initial_data.get('is_owner', None)
+                    and self.model.stream_dict[self.message.get('stream_id')].
+                        get('stream_post_policy', None) == 2)):
+                    self.msg_footer = (
+                        self.model.controller.view.set_footer_text(
+                            "You are not authorised to send messages "
+                            "to this stream.",
+                            5))
+                else:
+                    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(