artists: fix editing invalid urls in artist entries (fix #3720, #3927, #3781)

Convert to an autosave association on urls. This ensures that when we
save the artist we only validate the added urls, not bad urls that we're
trying to remove, and that url validation errors are propagated up to
the artist object.

This also fixes invalid urls being saved in the artist history despite
validation failing (#3720).

test/unit/artist_test.rb

@@ -166,9 +166,19 @@ class ArtistTest < ActiveSupport::TestCase
     end
 
     should "not allow invalid urls" do
-      artist = FactoryBot.create(:artist, :url_string => "blah")
+      artist = FactoryBot.build(:artist, :url_string => "blah")
       assert_equal(false, artist.valid?)
-      assert_equal([" blah must begin with http:// or https://"], artist.errors[:url])
+      assert_equal(["'blah' must begin with http:// or https:// "], artist.errors["urls.url"])
+    end
+
+    should "allow fixing invalid urls" do
+      artist = FactoryBot.build(:artist)
+      artist.urls << FactoryBot.build(:artist_url, url: "www.example.com", normalized_url: "www.example.com")
+      artist.save(validate: false)
+
+      artist.update(url_string: "http://www.example.com")
+      assert_equal(true, artist.valid?)
+      assert_equal("http://www.example.com", artist.urls.map(&:to_s).join)
     end
 
     should "make sure old urls are deleted" do