user integration spec

A full stack, no internal mocks integration spec for the per user notifications that @pedro added recently.
> why test a match on all users, if there's only one user?

It's not a good reason, but the reason is I copied a lot from the other integration test

> This also tests the happy path only, so I'd create a second user and verify that the notification's user_ids don't match that other user.

The second, unused user is a good idea. I'll add that. This is though intentionally happy path only, as the endpoint tests do more failure testing. These are intended to make sure all the wiring between all the layers is good.