FEATURE: add support for responsive images in posts

When creating lightboxes we will attempt to create 1.5x and 2x thumbnails
for retina screens, this can be controlled with a new hidden site setting
called responsice_post_image_sizes, if you wish to create 3x images run

SiteSetting.responsive_post_image_sizes = "1|1.5|2|3"


The default should be good for most of the setups as it balances filesize
with quality. 3x thumbs can get big.

config/site_settings.yml

@@ -914,6 +914,9 @@ files:
     default: 'gz'
     type: list
     list_type: compact
+  responsive_post_image_sizes:
+    default: "1|1.5|2"
+    hidden: true
   crawl_images:
     default: true
   max_image_width: