don't hide case id column when case name is checked in form exports

this is just some annoying artifact of how couchexport deals with
columns with duplicate indexes in the export config UI.

fixes http://manage.dimagi.com/default.asp?105763
import
make constant for frequently used string
Merge pull request #3077 from dimagi/case-105763

don't hide case id column when case name is checked in form exports