also update cache logic to decode new jwt format of reddit_session

Author: eritbh

extension/data/background/handlers/cache.js

@@ -95,10 +95,9 @@ async function getSessionUserID (sender) {
     }
 
     if (redditSessionCookie) {
-        // The session value contains comma seperated values. The first one is the the userid in base10.
-        // As reddit uses base36 everywhere else we convert the ID to that so things are easier to debug.
-        const redditUserIdBase10 = decodeURIComponent(redditSessionCookie.value).match(/\d+/)[0];
-        redditUserIdBase36 = parseInt(redditUserIdBase10).toString(36);
+        // this cookie is a JWT now, fun. decode its payload and get the
+        // `sub`ject, removing `t3_`
+        redditUserIdBase36 = JSON.parse(atob(redditSessionCookie.value.split('.')[1])).sub.slice(3);
     } else {
         redditUserIdBase36 = 'noSessionFallback';
     }