Post History

**Identify and protect key slugs**

Some help topics are linked to from within the software.  For example, the search page links to the topic for advanced search, formatting help is linked when creating a post, and I think there's a warning about images without "alt" text that links to that topic.  I think there are a couple more cases.

Communities can edit their help, including deleting or refactoring topics.  What happens if a community changes or removes one of these special pages, so that those built-in links are no longer meaningful?  At the moment, people editing help don't see any cautions about this and we don't protect those URL slugs from being changed.  Can we prevent accidental breakage?

We'll need to compile a list of help pages that have these extra dependencies.  We should make that information available (somehow), and perhaps we should "lock" those URL slugs and prevent those pages from being deleted.  A message could be shown when editing, to alert editors to the dependencies.

I don't think we should lock these pages down entirely.  Communities have already extended the original formatting help, for example.  We want communities to be able to adjust help in the ways that meet their own needs -- we just want to avoid accidentally breaking dependencies.