Improvements for ordering in custom exports

http://manage.dimagi.com/default.asp?86347#695228
On [line 233](https://github.com/lwyszomi/commcare-hq/blob/86347-2/corehq/apps/export/templates/export/customize_export.html#L233) there is another iteration over `self.table_configuration()`. Would it make sense to move the body of this function into that iteration?