[mb-style] RFC: volume numbers normalization only for release groups

[Nikki]

It's been more than a week, you can send an RFV now.

Nikki

Philip Jägenstedt wrote:
> http://wiki.musicbrainz.org/User:Foolip/Volume_numbers
> 
> See the editing history for a clear diff. The essence of the change is
> to not normalize the release titles at all, only the release group
> titles. This is a similar split to track/recording.
>