Fix 9248: challenge creator should not automatically join their own challenge (#10383)

* fix(challenges): creator should not join challenge automatically

* change behavior on the client side as well

* update tests and fix membercount

* update tests

* fix tests

test/api/v3/integration/tasks/GET-tasks_challenge_challengeId.test.js

@@ -40,6 +40,7 @@ describe('GET /tasks/:taskId', () => {
     user = await generateUser();
     guild = await generateGroup(user);
     challenge = await generateChallenge(user, guild);
+    await user.post(`/challenges/${challenge._id}/join`);
   });
 
   it('returns error when incorrect id is passed', async () => {