compose: Fix resolved topic warning after send.

Previously, we did not clear the resolved topic warning when reopening
the compose box after sending a emssage. This was a latent bug before
the previous commit, since the resolve topic was always displayed when
the compose box was pointed at a resolved topic before the previous
commit.

static/js/compose.js

@@ -185,6 +185,7 @@ export function clear_compose_box() {
     }
     $("#compose-textarea").val("").trigger("focus");
     compose_validate.check_overflow_text();
+    compose_validate.clear_topic_resolved_warning();
     $("#compose-textarea").removeData("draft-id");
     compose_ui.autosize_textarea($("#compose-textarea"));
     $("#compose-send-status").hide(0);