FEATURE: Add thumbnails for chat image uploads (#24328)

Introduces the concept of image thumbnails in chat, prior to this we uploaded and used full size chat images within channels and direct messages.

The following changes are covered:
- Post processing of image uploads to create the thumbnail within Chat::MessageProcessor
- Extract responsive image ratios into CookedProcessorMixin (used for creating upload variations)
- Add thumbnail to upload serializer from plugin.rb
- Convert chat upload template to glimmer component using .gjs format
- Use thumbnail image within chat upload component (stores full size img in orig-src data attribute)
- Old uploads which don't have thumbnails will fallback to full size images in channels/DMs
- Update Magnific lightbox to use full size image when clicked
- Update Glimmer lightbox to use full size image (enables zooming for chat images)

lib/cooked_post_processor.rb

@@ -222,15 +222,6 @@ class CookedPostProcessor
     end
   end
 
-  def each_responsive_ratio
-    SiteSetting
-      .responsive_post_image_sizes
-      .split("|")
-      .map(&:to_f)
-      .sort
-      .each { |r| yield r if r > 1 }
-  end
-
   def optimize_image!(img, upload, cropped: false)
     w, h = img["width"].to_i, img["height"].to_i
     onebox = img.ancestors(".onebox, .onebox-body").first