[GREEN] Change cases CSV file name to have current datetime

apps/exportables/views.py

 from django.http import JsonResponse
 from django.db.models import Max
+from django.utils import timezone
 from rest_framework import status, serializers
 from rest_framework.response import Response
 from rest_framework.views import APIView
@@ -128,6 +129,7 @@ class ExportInvestigationCaseView(APIView):
                 "reference_case",
                 "author").values(
                 *INVESTIGATION_CASE_RENDERER_FIELDS))
-        headers = {"Content-Disposition": 'attachment; filename="cases.csv"'}
+        filename = "cases-{}.csv".format(timezone.localtime(timezone.now()))
+        headers = {"Content-Disposition": 'attachment; filename="{}"'.format(filename)}
 
         return Response(data, headers=headers)