Assets manager URLs don't get replaced

[mdonadio]

Grav version: 1.3.10
CDN version: 1.4.0

If a Twig template uses the asset manager, eg

{% block stylesheets %}
    {% do assets.addCss('theme://css/style-dist.css') %}
{% endblock %}
{{ assets.css() }}

then the final URLs don't get updated to point to the CDN. This appears to happen both with the pipeline on and off.

[mdonadio]

This may be the same root cause as #12, but created separate issues in case they aren't.

[mdonadio]

This is indeed the same problem as #12.