Define and enforce a policy for introducing redirects when site pages are deleted

[chalin]

Generally speaking, when pages disappear, we should add an appropriate redirect rule. (Related: #635)

Originally posted by @chalin in #643 (comment)

[austinlparker]

Let's tackle writing up the policy here as part of the revamped IA.

[chalin]

It would also be nice if we had checks to detect when redirects are missing. (I have some thoughts about how to do this.) Because it's easy to forget, e.g., #1098 (comment).