Fix up Github webhook creation

As noted in #279

I think the Github webhook creation is broken. I'm not able to do "Add Webhook". I see that the March challenge commit has some changes on the code related to webhook creation, it might be related to it.

https://github.com/topcoder-platform/topcoder-x-ui/commit/acd5be31a3e782417216c40c0e56df0f12986a10#diff-fbd58dcb765282ff93e051fdc36265beL314

Final Submission Guidelines

Git issue link: https://github.com/topcoder-platform/topcoder-x-ui/issues/304