Discontinue redirecting admins to non-AMP version when kept invalid markup
Discontinue redirecting admins to non-AMP version when kept invalid markup

For non-admins, redirect to non-AMP with a special value for the noamp query var that prevents AMP from even being made available (so there is no link to try to go back to the AMP version).

Show the status of the validation errors in the admin bar.

Fixes #4626

ampproject/amp-toolbox-php,ampproject/amp-toolbox-php