Fix "sort by" dropdown being empty

Must be something in the latest version of Chrome that caused this to start being a problem, because @franzliedke started experiencing it a few days ago, and I only just experienced it for the first time yesterday.

js/forum/src/components/IndexPage.js

@@ -213,7 +213,7 @@ export default class IndexPage extends Page {
     items.add('sort',
       Select.component({
         options: sortOptions,
-        value: this.params().sort,
+        value: this.params().sort || Object.keys(app.cache.discussionList.sortMap())[0],
         onchange: this.changeSort.bind(this)
       })
     );