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

app/main/views/jobs.py

@@ -198,7 +198,8 @@ def view_notifications(service_id, message_type):
         partials=get_notifications(service_id, message_type),
         message_type=message_type,
         status=request.args.get('status'),
-        page=request.args.get('page', 1)
+        page=request.args.get('page', 1),
+        to=request.args.get('to'),
     )
 
 
@@ -241,7 +242,9 @@ def get_notifications(service_id, message_type, status_override=None):
         page=page,
         template_type=[message_type],
         status=filter_args.get('status'),
-        limit_days=current_app.config['ACTIVITY_STATS_LIMIT_DAYS'])
+        limit_days=current_app.config['ACTIVITY_STATS_LIMIT_DAYS'],
+        to=request.args.get('to'),
+    )
 
     url_args = {
         'message_type': message_type,