fix thread.js not passing 'since' correctly to post updates api

data/static/js/thread.js

@@ -64,7 +64,7 @@
 
   function tryFetchUpdate() {
     if (!threadEndpoint) return;
-    const body = JSON.stringify({since: now});
+    const body = JSON.stringify({'since': now});
     fetch(threadEndpoint, {method: "POST", headers: {"Content-Type": "application/json"}, body: body})
       .then(res => res.json())
       .then(json => {