Avoid serving dirty AMP until it is supported by the runtime

[westonruter]

Since dirty AMP is not supported yet (see ampproject/amphtml#15583), on native AMP sites (where there is no non-AMP version) we should go ahead and auto-accept (sanitize) validation errors to remove the invalid AMP markup from the page. This could be simply implemented via:

if ( current_theme_supports( 'amp' ) ) {
    add_filter( 'amp_validation_error_sanitized', '__return_true' );
}

However what also needs to be done is that the validation errors need to always be shown in the editor instead of suppressing them once accepted.

This would basically mean that this logic would not normally run:

https://github.com/Automattic/amp-wp/blob/90acca403917c2e7b7f1b5985b03bdee7ffc6697/includes/class-amp-theme-support.php#L1141-L1153

The invalid AMP URL page should show a special warning when in native AMP instead of what is currently displayed:

This site is configured to automatically decide whether or not some validation errors may be accepted. For these errors, you cannot change their status below.

[westonruter]

/cc @ericlindley-g

[westonruter]

This also means we'll need to prevent AMP-specific Gutenberg blocks from being registered when not in native AMP mode.