Warn site admins when configuration loaded via file is out of sync with what is in the DB

Created by: slimsag

When config is loaded via file (critical, site, or external service config) and we perform automatic migrations on it as part of a Sourcegraph upgrade the site admin needs to know that they should visit the config pages and copy the (now migrated) configuration back into their JSON files.

Today, this process is manual and relies on us either telling them to do this or them doing it every time (which is tedious and very manual). It would be better if we simply detected this and told the site admin they should do so.

If this is something you want, please let us know!