compose: Show banner to explain non interleaved view messages fading.

In a non interleaved view when composing a message to another
conversation we fade messages which the user is not replying to,
to reduce the chance they send a message to a recipient they didn't
intend to. Also, it reduces the visual/cognitive processing required
to figure out where their message is going to go.

But, it's not necessarily clear to users that what the
fading means, so this commit adds a one-time compose banner
to explain what's going on the first time this comes up.

Fixes part of #29076.

web/src/compose_actions.ts

@@ -407,6 +407,7 @@ export function cancel(): void {
     hide_box();
     clear_box();
     compose_banner.clear_message_sent_banners();
+    compose_banner.clear_non_interleaved_view_messages_fading_banner();
     call_hooks(compose_cancel_hooks);
     compose_state.set_message_type(undefined);
     compose_pm_pill.clear();