Issue #3633586 by d.steindl: <none> in the 403/404 defaults gets translated by locale

The URL tags' values are schema type text without a translation context, so locale looked up the shipped <none> ("do not output the tag") under the interface string <none> and replaced it with its translation (e.g. Dutch -geen-), which was then output as the canonical and shortlink URL.

canonical_url and shortlink get the translation context 'Metatag URL': still translatable, but never by an interface string. A post update puts <none> back where the value is exactly the locale translation of <none>, in the active configuration and in language overrides.

Closes #3633586