compose: Prevent lingering textarea typeahead after sending message.

Compose textarea typeaheads don't hide when `Send` button is clicked.
If you type `@` and press send, mention typeahead is still visible.
This happens since the textarea is still focused after sending the
message while other input area trigger a `blur` event which hide
the typeahead for them.

We always call `clear_compose_box` wherever textarea is cleared
as a result of sending message. So, it is the right function to
hide the typeahead in.

web/src/compose.js

@@ -130,6 +130,7 @@ export function clear_compose_box() {
         compose_ui.make_compose_box_original_size();
     }
     $("textarea#compose-textarea").val("").trigger("focus");
+    compose_ui.compose_textarea_typeahead?.hide();
     compose_validate.check_overflow_text();
     compose_validate.clear_topic_resolved_warning();
     drafts.set_compose_draft_id(undefined);