drafts: Fix hook for saving drafts.

Previously, the hook to save drafts when closing the compose box was a
focusout on #compose-textarea. This mostly worked, and was essentially
invisible to users (because the animation to notify you about saved
drafts was in the close_compose_ui widget that is hidden), but you'd
end up seeing the tooltip appear (in the wrong place) if you closed
the compose box immediately after sending a message with Tab+Enter.

The correct fix is for the drafts hook to be just before we start
clearing compose box state, inside hide_box.

This was difficult to catch in a development environment, since one
doesn't use that flow with "Enter sends" enabled.

frontend_tests/node_tests/drafts.js

@@ -174,10 +174,6 @@ test("initialize", (override) => {
     };
 
     drafts.initialize();
-
-    const message_content = $("#compose-textarea");
-    assert.equal(message_content.get_on_handler("focusout"), drafts.update_draft);
-    message_content.trigger("focusout");
 });
 
 test("remove_old_drafts", () => {