Add a limit so that users can't upload more if they already have more
than 250 images queued for upload.
For example, if you upload a Pixiv post that has 200 images, then you'll
have 200 queued images for upload. This will go down as the images are
processed. If you exceed the limit, then trying to create new uploads
will return an error.
This is to prevent single users from overwhelming the site by uploading
too many images at once, thereby preventing other users from uploading
because the job queue is backed up and can't process new uploads by
other users until existing uploads are finished.
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