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.

tests/app/v2/notifications/test_get_notifications.py

@@ -0,0 +1,46 @@
+import json
+
+from app import DATETIME_FORMAT
+from tests import create_authorization_header
+
+
+def test_get_notification_by_id_returns_200(client, sample_notification):
+    auth_header = create_authorization_header(service_id=sample_notification.service_id)
+
+    response = client.get(
+        path='/v2/notifications/{}'.format(sample_notification.id),
+        headers=[('Content-Type', 'application/json'), auth_header])
+
+    assert response.status_code == 200
+    assert response.headers['Content-type'] == 'application/json'
+
+    json_response = json.loads(response.get_data(as_text=True))
+
+    expected_template_response = {
+        'id': '{}'.format(sample_notification.serialize()['template']['id']),
+        'version': sample_notification.serialize()['template']['version'],
+        'uri': sample_notification.serialize()['template']['uri']
+    }
+
+    expected_response = {
+        'id': '{}'.format(sample_notification.id),
+        'reference': None,
+        'email_address': None,
+        'phone_number': '{}'.format(sample_notification.to),
+        'line_1': None,
+        'line_2': None,
+        'line_3': None,
+        'line_4': None,
+        'line_5': None,
+        'line_6': None,
+        'postcode': None,
+        'cost': sample_notification.cost(),
+        'type': '{}'.format(sample_notification.notification_type),
+        'status': '{}'.format(sample_notification.status),
+        'template': expected_template_response,
+        'created_at': sample_notification.created_at.strftime(DATETIME_FORMAT),
+        'sent_at': sample_notification.sent_at,
+        'completed_at': sample_notification.completed_at()
+    }
+
+    assert json_response == expected_response

tests/app/v2/notifications/test_notification_schemas.py

@@ -4,8 +4,9 @@ import pytest
 from flask import json
 from jsonschema import ValidationError
 
-from app.v2.notifications.notification_schemas import post_sms_request, post_sms_response, post_email_request, \
-    post_email_response
+from app.v2.notifications.notification_schemas import (
+    post_sms_request, post_sms_response, post_email_request, post_email_response
+)
 from app.schema_validation import validate
 
 valid_json = {"phone_number": "07515111111",