stream_settings: Add new disable_topics option to topics_policy.

Adds new configuration option `disable_topics` in `topics_policy`
channel setting to support disabling topics in the channel.

Fixes #34553.

tools/linter_lib/custom_check.py

@@ -250,6 +250,7 @@ python_rules = RuleList(
             "exclude": FILES_WITH_LEGACY_SUBJECT,
             "exclude_line": {
                 ("zerver/lib/message.py", "message__subject__iexact=message.topic_name(),"),
+                ("zerver/lib/streams.py", '.exclude(subject="")'),
                 ("zerver/views/streams.py", "message__subject__iexact=topic_name,"),
                 ("zerver/lib/message_cache.py", 'and obj["subject"] == ""'),
                 (