Implement more image resizing, fix responsiveness

- Two new image sizes have been added: xl and xxl.
- Image resizing has been implemented in featured images.
- Some elements were not responsive, such as the link preview and code block.

partials/post-card-big.hbs

@@ -2,7 +2,10 @@
     <a class="mind-post-card-big_main" href="{{url}}">
 
         {{#if feature_image}}
-            <img class="mind-post-card-big_feature-image" src="{{img_url feature_image size="m"}}" alt="Feature Image"/>
+            <img class="mind-post-card-big_feature-image" src="{{img_url feature_image size="m"}}" alt="Feature Image"
+                 srcset="{{img_url feature_image size="s"}} 300w,
+                         {{img_url feature_image size="m"}} 600w"
+            />
         {{/if}}
         <header class="mind-post-card-big_header">
             <p class="mind-post-card-big_meta"><span class="mind-post-card-big_tag">{{primary_tag.name}}</span>