Create 'v2' get notification route

The new 'v2' API wants to return less data than the previous one,
which was sending back tons of fields the clients never used.

This new route returns only useful information, with the JSON
response dict being built up in the model's `.serialize()` method.

Note that writing the test for this was a bit painful because of
having to treat loads of keys differently. Hopefully we think this
is a good way to write this test, because if we don't, we should
start thinking of a better way to check the values are what we
expect.

app/v2/notifications/get_notifications.py

@@ -1,9 +1,17 @@
+from flask import jsonify
+
+from app import api_user
+from app.dao import notifications_dao
 from app.v2.notifications import notification_blueprint
 
 
 @notification_blueprint.route("/<uuid:id>", methods=['GET'])
 def get_notification_by_id(id):
-    pass
+    notification = notifications_dao.get_notification_with_personalisation(
+        str(api_user.service_id), id, key_type=None
+    )
+
+    return jsonify(notification.serialize()), 200
 
 
 @notification_blueprint.route("/", methods=['GET'])