Reconsider post-verification behavior of /unverified URLs

Our current design is such that:

1. For a name with no verified authors, `people/name-slug` redirects to `people/name-slug/unverified` to emphasize that the papers may belong to multiple individuals.
2. If one author for `name-slug` is then verified, and the name is not marked as ambiguous, the page moves to `people/name-slug` and `people/name-slug/unverified` becomes a 404.
3. If the name becomes ambiguous, `people/name-slug/unverified` then lists any papers that have not been disambiguated.

This means that a URL can easily go from serving content to 404 and back to serving content. This could cause whiplash for search engines (see #8642) and also confusion for users who have an old unverified URL.

Some possible changes:

- If all papers for a name are on a verified author page, instead of serving a 404 for `people/name-slug/unverified`, redirect to the verified page.
* This is simple but does not remove whiplash. Inverting the redirects such that X->Y becomes Y->X seems dangerous, esp. considering that search engines index different URLs at different times.
- Instead of 404 for `people/name-slug/unverified`, serve a page that links to the verified author page as canonical and suggests its URL.
* Suggested by @lgessler: `` tells search engines that this is not the main page to show in search results anymore. Telling the user "please use this other URL from now on" is more informative/less frustrating than a 404. The page title could be something like "Matt Post (redirect)".
* The effect is that `/unverified` URLs would be permanent, but discouraged if there is a single verified author with the name.
* I don't know if the `` solution would force indexers to reindex `people/name-slug`.