Label the search box dependent on message type

It doesn’t make sense to say ‘Search by email address or phone number’
when you’re only looking at emails.

app/main/views/jobs.py

@@ -183,7 +183,10 @@ def view_notifications(service_id, message_type=None):
         status=request.args.get('status') or 'sending,delivered,failed',
         page=request.args.get('page', 1),
         to=request.form.get('to', ''),
-        search_form=SearchNotificationsForm(to=request.form.get('to', '')),
+        search_form=SearchNotificationsForm(
+            message_type=message_type,
+            to=request.form.get('to', ''),
+        ),
         download_link=url_for(
             '.download_notifications_csv',
             service_id=current_service.id,