[$50] Sanitise request body

[ThomasKranitsas]

Based on #293 (comment) we need to use https://www.npmjs.com/package/express-sanitizer to sanitize the request body wherever needed.

cc @rootelement

[ThomasKranitsas]

The recommended package was not very convenient to use so I used xss instead.

PR #310

[rootelement]

Contest https://www.topcoder.com/challenges/30143956 has been created for this ticket.

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[rootelement]

Contest https://www.topcoder.com/challenges/30143956 has been updated - it has been assigned to thomaskranitsas.

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[rootelement]

Payment failed: [500]: Failed to get challenge details by Id

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