Fix custom_range bugs with ugettext_lazy in custom range object name,
and unique fields error when creating existing custom ranges.
Fix for delegate of num_products to a proxy class as documented.
Ensure custom range names are text

Rather than forcing text, it's better to raise an error when a uggettext
proxy is used for the range name as this won't save correctly in the
database.

Related to #1053
Merge pull request #1053