make copyright date the posted date

really should include created as well, but that would make things more
complicated, or maybe have a header that overrides the copyright year..

layout/blog.j2

@@ -55,4 +55,4 @@
 
 {%- endblock %}
 
-{% block copyright -%}Copyright © {{ resource.meta.created|date_format("%Y") }} John-Mark Gurney{%- endblock %}
+{% block copyright -%}Copyright © {{ resource.meta.posted|date_format("%Y") }} John-Mark Gurney{%- endblock %}