Add an option to host CC icon rather than querying an external service

See #112 for more context on this.

templates/partial/cc_license.html

@@ -9,7 +9,11 @@
     <img alt="Creative Commons License"
          title="Creative Commons License"
          style="border-width:0"
+         {% if HOST_CC_ICON %}
+           src="{{ SITEURL }}/{{ THEME_STATIC_DIR }}/img/cc/{{ CC_LICENSE['slug'] }}.png"
+         {% else %}
            src="https://i.creativecommons.org/l/{{ CC_LICENSE['slug'] }}/{{ CC_LICENSE['version'] }}/80x15.png"
+         {% endif %}
          width="80"
          height="15"/>
   </a>