custom fields for results and assignments

- added custom field capability to results and assignments
- added ability to delete custom fields from items
- moved find parent functions to helpers file for stencils, test cases,
test plans
- added new link type custom field
- added db migration files for adding custom fields to results and
assignments during setup script
Merge pull request #53 from agramian/custom-fields-for-more-item-types

custom fields for results and assignments