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/sidebar.html

@@ -5,7 +5,7 @@
     <a href="{{ '/' | relative_url }}" id="avatar" class="rounded-circle">
       {%- if site.avatar != empty and site.avatar -%}
         {%- capture avatar_url -%}
-          {% include img-url.html src=site.avatar %}
+          {% include media-url.html src=site.avatar %}
         {%- endcapture -%}
         <img src="{{- avatar_url -}}" width="112" height="112" alt="avatar" onerror="this.style.display='none'">
       {%- endif -%}