Don't destroy discussion list in case of a redraw

i.e. the one in index-page’s onunload → composer.minimize

js/forum/src/components/index-nav-item.js

@@ -3,7 +3,9 @@ import NavItem from 'flarum/components/nav-item'
 export default class IndexNavItem extends NavItem {
   static props(props) {
     props.onclick = props.onclick || function() {
-      app.cache.discussionList = null;
+      if (app.cache.discussionList) {
+        app.cache.discussionList.forceReload = true;
+      }
       m.redraw.strategy('none');
     };
   }