DS-1945 RequestItem Helpdesk has options to contact requester, and author
DS-1945 Add request_message to requestitem DB table

Also, refactored requestitem to be a dspace-api "model", moved all DB logic
from XMLUI regarding this module to the dspace-api model.

Add the requestitem.request_message to display in the contact-author form,
so they can see what the original request message was.