Disable cropped (square) thumbnails on mobile. Use regular uncropped
thumbnails instead.
This is for a few reasons:
* It made it harder to support multiple thumbnail sizes or file formats,
since we need a cropped and uncropped version for every size and format.
* The cropping algorithm wasn't that great and sometimes cropped out
important parts of the image.
* The thumbnail type (cropped or uncropped) was automatically chosen
clientside based on the user's screen size, which made certain things
like adjustable thumbnail sizes more difficult because we didn't know
which thumbnail type the user actually had.
This may return again as an option in the future, but for now it's disabled.
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