Fix #4470: Check URLs for duplicates when creating artists

Show a warning when creating a duplicate artist; that is, when adding a
URL that already belongs to another artist.

This is a soft warning rather than a hard error because there are some
cases where multiple artists legitimately share the same site or account.

app/models/artist.rb

@@ -15,6 +15,7 @@ class Artist < ApplicationRecord
 
   validate :validate_artist_name
   validates :name, tag_name: true, uniqueness: true
+  after_validation :add_url_warnings
 
   before_save :update_tag_category
   after_save :create_version
@@ -302,6 +303,12 @@ class Artist < ApplicationRecord
     end
   end
 
+  def add_url_warnings
+    urls.each do |url|
+      warnings.add(:base, url.warnings.full_messages.join("; ")) if url.warnings.any?
+    end
+  end
+
   include UrlMethods
   include VersionMethods
   extend FactoryMethods