Create 'v2' get notifications route

Return multiple notifications for a service.
Choosing a page_size or a page_number is no longer allowed.
Instead, there is a `next` link included with will return the
next {default_page_size} notifications in the sequence.

Query parameters accepted are:

 - template_type: filter by specific template types
 - status: filter by specific statuses
 - older_than: return a chronological list of notifications older
   than this one. The notification with the id that is passed in
   is _not_ returned.

Note that both `template_type` and `status` can accept multiple
parameters.  Thus it is possible to call
`/v2/notifications?status=created&status=sending&status=delivered`

app/dao/notifications_dao.py

@@ -262,7 +262,8 @@ def get_notifications_for_service(
     key_type=None,
     personalisation=False,
     include_jobs=False,
-    include_from_test_key=False
+    include_from_test_key=False,
+    older_than=None
 ):
     if page_size is None:
         page_size = current_app.config['PAGE_SIZE']
@@ -273,6 +274,11 @@ def get_notifications_for_service(
         days_ago = date.today() - timedelta(days=limit_days)
         filters.append(func.date(Notification.created_at) >= days_ago)
 
+    if older_than is not None:
+        older_than_created_at = db.session.query(
+            Notification.created_at).filter(Notification.id == older_than).as_scalar()
+        filters.append(Notification.created_at < older_than_created_at)
+
     if not include_jobs or (key_type and key_type != KEY_TYPE_NORMAL):
         filters.append(Notification.job_id.is_(None))