Update the challenge object after forum created in processor

After we've created the group / forum for a challenge, we need to update the challenge object to contain a reference back to the forum created:

So you should be able to POST/PUT/PATCH with the challenge object as laid out in this git issue: https://github.com/topcoder-platform/challenge-api/issues/290

More information on the format is here: https://github.com/topcoder-platform/challenge-api/issues/290#issuecomment-696921827

From Topcoder:

And you should assume that there can be multiple vanilla forums for whatever reason. So you should loop through all the challenge objects in the array, and if they���re type: challenge, and provider: vanilla, and the url is empty, create that forum.

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Git issue link: https://github.com/topcoder-platform/forums/issues/51