Update links schema to be more strict and fix import

tests/app/notifications/rest/test_callbacks.py

@@ -3,7 +3,7 @@ import uuid
 from datetime import datetime
 from flask import json
 from freezegun import freeze_time
-from mock import call
+from unittest.mock import call
 
 import app.celery.tasks
 from app.dao.notifications_dao import (

tests/app/public_contracts/schemas/GET_notifications_return.json

@@ -20,6 +20,9 @@
         },
         "next" : {
           "type" : "string"
+        },
+        "prev" : {
+          "type" : "string"
         }
       }
     },