compose_banner: Remove uploads banners when clearing compose box.

Upload banners were not cleared after closing compose box, which meant
that they would remain present in a paused state after compose was reopened.

https://chat.zulip.org/#narrow/stream/9-issues/topic/Incomplete.20Upload.20banner.20remains.20on.20closing.20compose/near/1582602

web/src/compose.js

@@ -187,6 +187,7 @@ export function clear_compose_box() {
     compose_ui.autosize_textarea($("#compose-textarea"));
     compose_banner.clear_errors();
     compose_banner.clear_warnings();
+    compose_banner.clear_uploads();
     compose_ui.hide_compose_spinner();
     popover_menus.reset_selected_schedule_timestamp();
 }