request: Refactor ZulipRequestNotes to RequestNotes.

This utilizes the generic `BaseNotes` we added for multipurpose
patching. With this migration as an example, we can further support
more types of notes to replace the monkey-patching approach we have used
throughout the codebase for type safety.

zerver/webhooks/slack/view.py

@@ -5,7 +5,7 @@ from django.utils.translation import gettext as _
 from zerver.decorator import webhook_view
 from zerver.lib.actions import check_send_stream_message
 from zerver.lib.exceptions import JsonableError
-from zerver.lib.request import REQ, get_request_notes, has_request_variables
+from zerver.lib.request import REQ, RequestNotes, has_request_variables
 from zerver.lib.response import json_success
 from zerver.models import UserProfile
 
@@ -35,7 +35,7 @@ def api_slack_webhook(
         subject = _("Message from Slack")
 
     content = ZULIP_MESSAGE_TEMPLATE.format(message_sender=user_name, text=text)
-    client = get_request_notes(request).client
+    client = RequestNotes.get_notes(request).client
     assert client is not None
     check_send_stream_message(user_profile, client, stream, subject, content)
     return json_success()