Prevent duplicate challenge tasks (#11502)

* Implement atomic user challenges update

Prevents multiple concurrent requests from passing through
Fixes #11295

* Move user challenges update to separate method

* Rename challenge syncToUser to syncTasksToUser

Now that adding the challenge to user is separated, this methods main purpose is to sync the tasks

* Fix lint errors

migrations/challenges/sync-all-challenges.js

@@ -10,7 +10,7 @@ async function syncChallengeToMembers (challenges) {
 
     const promises = [];
     users.forEach(user => {
-      promises.push(challenge.syncToUser(user));
+      promises.push(challenge.syncTasksToUser(user));
       promises.push(challenge.save());
       promises.push(user.save());
     });