FIX: correct upload statistics report for external storage

Follows up #64b35120

This also corrects it so bytes used for internal storage counts all the space
used, previously it was only counting uploads not optimized images.

Additionally we now correctly count storage for optimized images.

lib/disk_space.rb

@@ -2,13 +2,19 @@
 
 class DiskSpace
   def self.uploads_used_bytes
-    # used(uploads_path)
-    # temporary (on our internal setup its just too slow to iterate)
-    Upload.sum(:filesize).to_i
+    if Discourse.store.external?
+      Upload.sum(:filesize).to_i + OptimizedImage.sum(:filesize).to_i
+    else
+      used(uploads_path)
+    end
   end
 
   def self.uploads_free_bytes
-    free(uploads_path)
+    if Discourse.store.external?
+      0
+    else
+      free(uploads_path)
+    end
   end
 
   def self.free(path)