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.
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