added `create_thumbnails?` site setting

defaults to `false`

app/models/site_setting.rb

@@ -71,6 +71,7 @@ class SiteSetting < ActiveRecord::Base
   setting(:queue_jobs, !Rails.env.test?)
   setting(:crawl_images, !Rails.env.test?)
   setting(:max_image_width, 690)
+  setting(:create_thumbnails, false)
   client_setting(:category_featured_topics, 6)
   setting(:topics_per_page, 30)
   setting(:posts_per_page, 20)

config/locales/server.en.yml

@@ -583,6 +583,7 @@ en:
     newuser_max_mentions_per_post: "Maximum number of @name notifications a new user can use in a post"
     max_mentions_per_post: "Maximum number of @name notifications you can use in a post"
 
+    create_thumbnails: "Create thumbnails for lightboxed images"
     auto_link_images_wider_than: "Images wider than this, in pixels, will get auto link and lightbox treatment"
 
     email_time_window_mins: "How many minutes we wait before sending a user mail, to give them a chance to see it first"