artists: replace artist finder with fetch source data.

* 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.

app/javascript/src/javascripts/posts.js.erb

@@ -441,7 +441,7 @@ Post.initialize_post_sections = function() {
       $("#share").hide();
       $("#post_tag_string").focus().selectEnd().height($("#post_tag_string")[0].scrollHeight);
       $("#related-tags-button").trigger("click");
-      $("#find-artist-button").trigger("click");
+      $("#fetch-data-manual").trigger("click");
       $("#recommended").hide();
     } else if (e.target.hash === "#recommended") {
       $("#comments").hide();