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.

dspace/etc/postgres/database_schema.sql

@@ -836,6 +836,7 @@ CREATE TABLE requestitem
   allfiles bool,
   request_email varchar(64),
   request_name varchar(64),
+  request_message TEXT,
   request_date timestamp,
   accept_request bool,
   decision_date timestamp,