artists: normalize group names.

Normalize artist group names following the same rules as artist other names.

This means artist group names now use underscores instead of spaces.
It also means extra space characters at the beginning and end of names
is stripped, and Unicode characters are normalized.

Fixes #4647, which was caused by users accidentally replacing group
names with a single space character when trying to remove a group.

app/models/artist.rb

@@ -9,6 +9,7 @@ class Artist < ApplicationRecord
   deletable
 
   normalize :name, :normalize_name
+  normalize :group_name, :normalize_other_name
   normalize :other_names, :normalize_other_names
   array_attribute :other_names # XXX must come after `normalize :other_names`