request: Store client information using ZulipRequestNotes.

This concludes the HttpRequest migration to eliminate arbitrary
attributes (except private ones that are belong to django) attached
to the request object during runtime and migrated them to a
separate data structure dedicated for the purpose of adding
information (so called notes) to a HttpRequest.

zerver/lib/home.py

@@ -16,6 +16,7 @@ from zerver.lib.i18n import (
     get_language_list,
     get_language_translation_data,
 )
+from zerver.lib.request import get_request_notes
 from zerver.models import Message, Realm, Stream, UserProfile
 from zerver.views.message_flags import get_latest_update_message_flag_activity
 
@@ -138,9 +139,11 @@ def build_page_params_for_home_page_load(
     }
 
     if user_profile is not None:
+        client = get_request_notes(request).client
+        assert client is not None
         register_ret = do_events_register(
             user_profile,
-            request.client,
+            client,
             apply_markdown=True,
             client_gravatar=True,
             slim_presence=True,