fix(topics): Fix add topics from sidebar feature.

This feature broke due to a recent refactor. JSON stringifying now happens lower down in the call stack.

static/js/TopicSearch.jsx

@@ -64,7 +64,7 @@ class TopicSearch extends Component {
   }
 
   post(slug) {
-      const postJSON = JSON.stringify({"topic": slug, 'interface_lang': Sefaria.interfaceLang});
+      const postJSON = {"topic": slug, 'interface_lang': Sefaria.interfaceLang};
       const srefs = this.props.srefs;
       const update = this.props.update;
       const reset = this.reset;