sources: add methods for customizing new artist entries.

* Rename `unique_id` to `tag_name`.

* Add `other_names` and `profile_urls` methods that sources can override
  to provide extra names or urls when creating new artist entries.

app/logical/sources/strategies/null.rb

@@ -24,10 +24,6 @@ module Sources
       def normalize_for_artist_finder
         url
       end
-
-      def unique_id
-        url
-      end
     end
   end
 end