FEATURE: Show a blurry preview when lazy loading images

This generates a 10x10 PNG thumbnail for each lightboxed image.
If Image Lazy Loading is enabled (IntersectionObserver API) then
we'll load the low res version when offscreen. As the image scrolls
in we'll swap it for the high res version.

We use a WeakMap to track the old image attributes. It's much less
memory than storing them as `data-*` attributes and swapping them
back and forth all the time.

app/models/upload.rb

@@ -65,7 +65,8 @@ class Upload < ActiveRecord::Base
     opts = opts.merge(raise_on_error: true)
     begin
       OptimizedImage.create_for(self, width, height, opts)
-    rescue
+    rescue => ex
+      Rails.logger.info ex if Rails.env.development?
       opts = opts.merge(raise_on_error: false)
       if fix_image_extension
         OptimizedImage.create_for(self, width, height, opts)