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/img-url.html

@@ -0,0 +1,24 @@
+{%- comment -%}
+  Generate image final URL based on `site.img_cdn`, `page.img_path`
+
+  Arguments:
+    src - basic image path, required
+    img_path - relative path of image, optional
+
+  Return:
+    image URL
+{%- endcomment -%}
+
+{% assign url = include.src %}
+
+{%- if url -%}
+  {%- comment -%} CND URL {%- endcomment -%}
+  {% assign prefix = site.img_cdn | default: '' | relative_url %}
+
+  {%- comment -%} Add page image path prefix {%- endcomment -%}
+  {% assign url = include.img_path | default: '' | append: '/' | append: url %}
+
+  {% assign url = prefix | append: '/' | append: url | replace: '///', '/' | replace: '//', '/' | replace: ':', ':/' %}
+{%- endif -%}
+
+{{- url -}}