Prevent admins from saving site config if it doesn't pass validation

Created by: slimsag

A site admin clicked Add license key, which inserted:

"licenseKey": "<license key>"

They saved this, which worked fine, and then produces severe site config alerts everywhere.

It would be ideal if we prevented site config edits if they introduce validation problems.

@unknwon you've recently been working with the site config code, perhaps you'd be interested in this?