artists: improve prefilling of new artist form (#4028)

* When creating an artist by clicking the '?' next to the artist tag in
  the tag list, prefill the new artist form by finding the artist's last
  upload and fetching its source data.

  Previously we filled the urls with the source of the artist's last
  upload, which was wrong because it was usually a direct image URL (#3078).

* Fix the other names field not escaping spaces within names to underscores.

* Fix the other names field being potentially prefilled with duplicate names.

test/unit/artist_test.rb

@@ -550,5 +550,26 @@ class ArtistTest < ActiveSupport::TestCase
         assert_equal(1, @artist.urls.count)
       end
     end
+
+    context "#new_with_defaults" do
+      should "fetch the defaults from the given source" do
+        source = "https://i.pximg.net/img-original/img/2018/01/28/23/56/50/67014762_p0.jpg"
+        artist = Artist.new_with_defaults(source: source)
+
+        assert_equal("niceandcool", artist.name)
+        assert_equal("nice_and_cool", artist.other_names_string)
+        assert_equal("https://www.pixiv.net/member.php?id=906442", artist.url_string)
+      end
+
+      should "fetch the defaults from the given tag" do
+        source = "https://i.pximg.net/img-original/img/2018/01/28/23/56/50/67014762_p0.jpg"
+        FactoryBot.create(:post, source: source, tag_string: "test_artist")
+        artist = Artist.new_with_defaults(name: "test_artist")
+
+        assert_equal("test_artist", artist.name)
+        assert_equal("nice_and_cool", artist.other_names_string)
+        assert_equal("https://www.pixiv.net/member.php?id=906442", artist.url_string)
+      end
+    end
   end
 end