message_scroll: Use fadeOut effect to hide the scroll to bottom button.
message_scroll: Use fadeOut effect to hide the scroll to bottom button.

We use `fadeOut` effect to hide the scroll to bottom button more
widely. We already use `fadeIn` effect to display the button
every time.

We deliberately don't use the `fadeOut` effect when doing
`make_compose_box_full_size` to avoid any button overlap with
compose for a short time.

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