Updates to prevent retirees from displaying in directory

Note: This PR branch has been applied to both the Production (https://directory.unl.edu) and Test (https://directory-test.unl.edu) sites for testing.

Changes made:

Add affiliation constants
Prevent person records with only Retiree and Volunteer affiliations.
Prevent display of roles of Retirees Org Unit or with title/description of 'retiree'

I'm not sure if these changes will prevent all retirees or retiree information from displaying or if some valid person may be excluded, but it should handle most cases.

unl / UNL_Directory

Updates to prevent retirees from displaying in directory #191