Refactor the /related_tags.js call so that when the "Related tags"
button is clicked, it only replaces the columns that actually changed,
not the entire related tags section.
This avoids rebuilding the Recent / Frequent / Translated Tags / Artist
columns every time the "Related tags" button is clicked.
Instead of calling /sources.json and rendering the "Fetch source data"
box client-side in Javascript, call /sources.js so we can render the
html server-side.
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.
