Make the upload page automatically detect when a source URL has multiple images
and let the user choose which images to post.
For example, when uploading a Twitter or Pixiv post with more than one image, we
direct the user to a page showing a thumbnail for each image and letting
them choose which ones to post.
This is similar to the batch upload page, except we actually download each image
in the background, instead of just hotlinking or proxying the thumbnails through
our servers. This avoids various problems with proxying and makes new features
possible, like showing which images in the batch have already been posted.
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