request: Extract generic BaseNotes from RequestNotes.
request: Extract generic BaseNotes from RequestNotes.

The motive of adding `BaseNotes` was to support monokey patching
temporary attributes to objects (such as `.trigger` on `Message`) when
working on the django-stubs migration in #18777.

andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,andersk/zulip,kou/zulip,rht/zulip,rht/zulip,kou/zulip