Add redirects validation + fix issues with redirects.yml

Why is this change being made?

This MR adds a redirects validator to ensure that the defined redirects are all valid.

This is particularly important for the work in progress to migrate from Fastly to Cloudflare as the worker will process every request (including redirects) so any issue with redirects means the worker wouldn't be able to serve traffic so we want to make sure the redirects are valid.

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