refactor!: rename media-url file and related parameters (#1651)

- Changed variable `img_cdn` to `cdn` in site configuration file.
- Changed the variable defining the relative path of the image in the post from `img_url` to `media_subpath`

_includes/embed/audio.html

@@ -4,7 +4,7 @@
 
 {% unless src contains '://' %}
   {%- capture src -%}
-    {% include img-url.html src=src %}
+    {% include media-url.html src=src %}
   {%- endcapture -%}
 {% endunless %}