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).
* In the `Fetch source data` box, list the Danbooru artist tag(s) that
were found beside the artist name.
* Unify `Fetch source data` code that was duplicated between the
uploads page and the post show page.
