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.

static/js/compose_actions.js

@@ -31,6 +31,8 @@ export function blur_compose_inputs() {
 }
 
 function hide_box() {
+    // This is the main hook for saving drafts when closing the compose box.
+    drafts.update_draft();
     blur_compose_inputs();
     $("#stream-message").hide();
     $("#private-message").hide();