GH revalidation errors should be displayed on the same page rather than a new page

When revalidating Github webhook requests, if there is an error in validation then the request will display this error as JSON on a new page. This isn't user-friendly, so we should change how the revalidation POST is done to resolve this. To best resolve this, we should manually use fetch to post the form data and capture the response instead. This way, handling of the errors can be done on the same page in a way that is more intuitive to users.