site: validate updated site configs

Created by: bobheadxi

closes #10883

Adds a site config validation step before accepting input. The validation doesn't correctly check for licence key values (specifically mentioned in #10883), so I've added a super simple check to error on the placeholder value. It's a little awkward - especially with a word like "license" which I often spell "licence", an exact string match is finicky. However, it doesn't look like there's a function for superficially validating a licence key outside of enterprise/internal, so I'm not sure if there's a better way to do this check 🤔 license key verification to conf.Validate