Defer clearing discussion list on discussion start

This prevents an edge case where `app.discussions` is considered empty while the new page is loading, and as a result, the side pane isn't set as "enabled". Then, if the pane has previously been pinned, when the page loads and the side pane appears, it covers up part of the discussion page.

Fixes https://github.com/flarum/core/issues/2471

js/src/forum/components/DiscussionComposer.js

@@ -100,7 +100,7 @@ export default class DiscussionComposer extends ComposerBody {
       .save(data)
       .then((discussion) => {
         this.composer.hide();
-        app.discussions.refresh();
+        app.discussions.refresh({ deferClear: true });
         m.route.set(app.route.discussion(discussion));
       }, this.loaded.bind(this));
   }