feat: add site-wide social preview image settings (#1463)

- Add configuration field `social_preview_image` that sets the site-wide default social preview image.  

   For pages that do not have `page.image` set in font matter, the seo tag will use the image specified by `site.social_preview_image` as the Open Graph image.

- Refactored the generation of image URLs to reduce redundant code and enhance fault tolerance for missing or repeated slash `/` when defining image paths.

_includes/refactor-content.html

@@ -48,23 +48,6 @@
   {% assign _img_content = null %}
   {% assign _img_snippets = _content | split: IMG_TAG %}
 
-  <!-- CDN URL -->
-  {% if site.img_cdn %}
-    {% if site.img_cdn contains '//' %}
-      {% assign _path_prefix = site.img_cdn %}
-    {% else %}
-      {% assign _path_prefix = site.img_cdn | relative_url %}
-    {% endif %}
-  {% else %}
-    {% assign _path_prefix = site.baseurl %}
-  {% endif %}
-
-  <!-- Add image path -->
-  {% if page.img_path %}
-    {% assign _path = page.img_path | append: '/' | replace: '//', '/' %}
-    {% assign _path_prefix = _path_prefix | append: _path %}
-  {% endif %}
-
   {% for _img_snippet in _img_snippets %}
     {% if forloop.first %}
       {% assign _img_content = _img_snippet %}
@@ -113,6 +96,12 @@
     {% assign _final_src = null %}
     {% assign _lazyload = true %}
 
+    {%- capture _img_url -%}
+      {% include img-url.html src=_src img_path=page.img_path %}
+    {%- endcapture -%}
+
+    {% assign _path_prefix = _img_url | remove: _src %}
+
     {% unless _src contains '//' %}
       {% assign _final_src = _path_prefix | append: _src %}
       {% assign _src_alt = 'src="' | append: _path_prefix %}