Let caseworkers send one off messages

The main task that we think ‘caseworker’ users do is send one off
messages.

So this commit:
- makes sure users who don’t have the `view_activity` permission (ie
  not ‘admin’ users) can still send messages
- adds navigation so that these users have a place to go from which to
  start the process of sending a one off message

tests/app/main/views/test_notifications.py

@@ -9,6 +9,8 @@ from notifications_python_client.errors import APIError
 
 from tests.conftest import (
     SERVICE_ONE_ID,
+    active_caseworking_user,
+    active_user_with_permissions,
     mock_get_notification,
     normalize_spaces,
 )
@@ -23,16 +25,23 @@ from tests.conftest import (
     ('permanent-failure', 'Phone number doesn’t exist'),
     ('technical-failure', 'Technical failure'),
 ])
+@pytest.mark.parametrize('user', [
+    active_user_with_permissions,
+    active_caseworking_user,
+])
 @freeze_time("2016-01-01 11:09:00.061258")
 def test_notification_status_page_shows_details(
     client_request,
     mocker,
     service_one,
     fake_uuid,
+    user,
     notification_status,
     expected_status,
 ):
 
+    mocker.patch('app.user_api_client.get_user', return_value=user(fake_uuid))
+
     _mock_get_notification = mock_get_notification(
         mocker,
         fake_uuid,