Read-only slug after initial publish

[mathiazom]

Slugs can now be changed easily via Sanity Studio. This risks killing shared links using older versions of the page slug.

Proposed solution: Make slug readonly after initial publish

Proposed solution:
Every time a slug is changed for a page, a redirection rule should be created to redirect the old to the new slug value.

Potential issues:
- Page A changes from /dog to /cat, then page B changes from /cat to /dog. Automatic redirection would redirect /dog to /cat, making page B inaccesible. Or page B would take presedence and the old url for page A would be broken. So a possible solution would be for active slug values to take presedence, while redirection is a fallback. Ideally a warning should be presented when there are conflicts.

[christinaroise]

I wonder if we should look into locking the slug as readonly once it's published. And only handle redirect in the redirect document

[mathiazom]

Relevant Sanity guide: https://www.sanity.io/guides/nextjs-automatic-redirects

[mathiazom]

Closed by #586