Bug: Uploading bad pixiv id images failed because the pixiv strategy
raised a BadIDError exception when the upload service checked for the
ugoira frame data.
Bug: Notes weren't rescaled when the 'final_source' field was given
during replacement.
The cause was that the notes were rescaled after the source was saved,
but saving the source clobbered `image_{width,height}_before_last_save`
inside `rescale_notes`.
Regressed in b0c2ddba.
* Rename Upload#download_for_upload to #get_file_for_upload.
* Fix #get_file_for_upload to raise error if no file or source url was given.
* Fix javascript upload validation to disallow submitting form if file is
not present and the source is not an url.
* On the /uploads/new page, instead of just showing a "This post has
probably already been uploaded" message, show the actual thumbnails of
posts having the same source as what the user is trying to upload.
* Move the iqdb results section up top, beside the related posts section.
Also makes it so that using the bookmarklet always triggers async upload
preprocessing. Before it was only triggered when the source passed a
dupe check, but that check was inaccurate (#3873).
