use service statistics endpoint instead of detailed service

we're not actually looking at the detailed service aspects - just
the stats. We're doing this in three places:

* dashboard
* notification activity page
* when checking jobs to see if we're over the daily limit

change these places to use a new api endpoint (service/id/statistics),
which hopefully be a little more performant, and will definitely be a
little more organised - moving away from generic endpoints with loads
of optional parameters.

We still need the detailed endpoints for the platform admin page tho.

Depends on https://github.com/alphagov/notifications-api/pull/1865

tests/app/main/views/test_accept_invite.py

@@ -371,7 +371,7 @@ def test_new_invited_user_verifies_and_added_to_service(
     mock_get_jobs,
     mock_has_permissions,
     mock_get_users_by_service,
-    mock_get_detailed_service,
+    mock_get_service_statistics,
     mock_get_usage,
     mock_create_event,
     mocker,