* On posts, automatically trigger "Fetch source data" when clicking the
Edit tab, instead of triggering the artist finder button. This way we
find both the artist and the translated tags in one ajax call.
* Remove the "Artist" finder button next to the source field. This isn't
necessary given that "Fetch source data" finds the artist itself.
* Remove the /artists/finder.json API endpoint. This is no longer used
after removing the "Artist" finder button.
Fixes#3807. Drops the TagScript module entirely and replaces it with
a simple call to Post.tag.
Simultaneously fixes#3773 as a consequence of removing TagScript.run.
Post.update already updates data-tags by itself, so there was no need
for TagScript.run to update data-tags as well.
