BUG Restrict URLSegment preview to editable fields
BUG Restrict URLSegment preview to editable fields

This fixes a bug where the Translatable module
would have a duplicate preview because it had
another ".urlsegment" field showing the value in its
original language.

See https://github.com/silverstripe/silverstripe-translatable/issues/43

silverstripe/silverstripe-cms,dhensby/silverstripe-cms,silverstripe-labs/silverstripe-reports,jonom/silverstripe-cms,jonom/silverstripe-cms,dhensby/silverstripe-cms,helpfulrobot/i-lateral-silverstripe-siteconfig,silverstripe/silverstripe-cms,dhensby/silverstripe-reports,silverstripe-labs/silverstripe-reports,dhensby/silverstripe-cms,dhensby/silverstripe-reports,silverstripe/silverstripe-siteconfig