Set section canonical url in 410 response

Currently we return a 410 when we encounter expired content. This is the correct thing to do but the Google web crawler apparently treats 410s just like 404s and so continually re-tests those urls which will in all likelihood never be available again.

By setting the canonical url to the parent category (section) of the 410-causing content, Google's crawler will learn not to re-try on the original url and so those 410s should eventually fade away from their index.

Always use `LinkTo{...}` when making URLs