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/nico_seiga.rb

@@ -112,7 +112,7 @@ module Sources
         "#{profile_url}/"
       end
 
-      def unique_id
+      def tag_name
         "nicoseiga#{api_client.user_id}"
       end