Expand API to allow setting USI Exemptions

A handful of government clients of ours have USI Exemptions across their organization. 

The current process is to create a report that monitors all enrollments from that organization and views their USI number, if its missing or they don't have an exemption it would be displayed. Periodically the report is ran and the USI exemptions are applied manually. 

Reviewing the API Docs, under /contact/ or /contact/:contactID resources there are no parameters to POST/PUT a USI Exemption.

Although you can view if a student has an exemption via the API.