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/controllers/artists_controller.rb

@@ -46,13 +46,14 @@ class ArtistsController < ApplicationController
   def create
     @artist = authorize Artist.new(permitted_attributes(Artist))
     @artist.save
+    flash[:notice] = [*@artist.errors.full_messages, *@artist.warnings.full_messages].join(".\n \n") if @artist.warnings.any? || @artist.errors.any?
     respond_with(@artist)
   end
 
   def update
     @artist = authorize Artist.find(params[:id])
     @artist.update(permitted_attributes(@artist))
-    flash[:notice] = @artist.valid? ? "Artist updated" : @artist.errors.full_messages.join("; ")
+    flash[:notice] = [*@artist.errors.full_messages, *@artist.warnings.full_messages].join(".\n \n") if @artist.warnings.any? || @artist.errors.any?
     respond_with(@artist)
   end