Remove duplication around MusicBrainz URL generation

lib/music_brainz/release_group.ex

@@ -14,6 +14,10 @@ defmodule MusicBrainz.ReleaseGroup do
     }
   end
 
+  def url(id) do
+    "https://musicbrainz.org/release-group/#{id}"
+  end
+
   defp parse_subtype("Album"), do: :album
   defp parse_subtype("EP"), do: :ep
   defp parse_subtype("Live"), do: :live