artists/edit: refactor editing nested wiki pages.

Refactor to use accepts_nested_attributes_for instead of the notes
attribute to facilitate editing wikis on the artist edit page.

This fixes the notes attribute unintentionally showing up in the API.

This also changes it so that renaming an artist entry doesn't
automatically rename the corresponding wiki page. This had bad behavior
when there was a conflict between wiki pages (the wikis would be
silently merged, which usually isn't what you want). It also didn't warn
about wiki links being broken by renames.

test/unit/artist_test.rb

@@ -104,24 +104,6 @@ class ArtistTest < ActiveSupport::TestCase
       end
     end
 
-    should "create a new wiki page to store any note information" do
-      artist = nil
-      assert_difference("WikiPage.count") do
-        artist = FactoryBot.create(:artist, :name => "aaa", :notes => "testing")
-      end
-      assert_equal("testing", artist.notes)
-      assert_equal("testing", artist.wiki_page.body)
-      assert_equal(artist.name, artist.wiki_page.title)
-    end
-
-    should "update the wiki page when notes are assigned" do
-      artist = FactoryBot.create(:artist, :name => "aaa", :notes => "testing")
-      artist.update_attribute(:notes, "kokoko")
-      artist.reload
-      assert_equal("kokoko", artist.notes)
-      assert_equal("kokoko", artist.wiki_page.body)
-    end
-
     should "normalize its name" do
       artist = FactoryBot.create(:artist, :name => "  AAA BBB  ")
       assert_equal("aaa_bbb", artist.name)