Challenge Listings: Challenge count

Only 15 challenges are displayed <img width="1440" alt="Screenshot 2020-05-19 at 6 06 29 PM" src="https://user-images.githubusercontent.com/58783823/82327083-8080ff00-99fb-11ea-95e1-d631acff6c9c.png">

Final Submission Guidelines

Git issue link: https://github.com/topcoder-platform/community-app/issues/4375