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/lib/home.py

@@ -15,7 +15,7 @@ from zerver.lib.i18n import (
     get_language_list,
     get_language_translation_data,
 )
-from zerver.lib.request import get_request_notes
+from zerver.lib.request import RequestNotes
 from zerver.models import Message, Realm, Stream, UserProfile
 from zerver.views.message_flags import get_latest_update_message_flag_activity
 
@@ -139,7 +139,7 @@ def build_page_params_for_home_page_load(
     }
 
     if user_profile is not None:
-        client = get_request_notes(request).client
+        client = RequestNotes.get_notes(request).client
         assert client is not None
         register_ret = do_events_register(
             user_profile,