Add query string parameter to search by recipient

> Service teams that use the admin interface often need to know the
> outcome of a message... at the moment they have to page through all
> the results in the activity stream. They should be able to find
> notifications by email address or phone number.

– https://www.pivotaltracker.com/n/projects/1443052

This commit adds an additional query string parameter (`to`) to the URL,
which users can use to filter down the list of notifications.

It:
- takes the status into account
- doesn’t update the counts based on the search term (in reality each
  service will only send a handful of notifications to one person in any
  7 day period)

In other words the funnel that filters down the notifications looks
like:

> all notifications for service → only failed → only to this phone
> number

tests/conftest.py

@@ -1069,6 +1069,7 @@ def mock_get_notifications(mocker, api_user_active):
         rows=5,
         include_jobs=None,
         include_from_test_key=None,
+        to=None,
     ):
         job = None
         if job_id is not None:
@@ -1101,7 +1102,9 @@ def mock_get_notifications_with_previous_next(mocker):
                            status=None,
                            limit_days=None,
                            include_jobs=None,
-                           include_from_test_key=None):
+                           include_from_test_key=None,
+                           to=None,
+                           ):
         return notification_json(service_id, with_links=True)
 
     return mocker.patch(