remove usage of notify_db fixture in unit tests

* notify_db fixture creates the database connection and ensures the test
  db exists and has migrations applied etc. It will run once per session
  (test run).
* notify_db_session fixture runs after your test finishes and deletes
  all non static (eg type table) data.

In unit tests that hit the database (ie: most of them), 99% of the time
we will need to use notify_db_session to ensure everything is reset. The
only time we don't need to use it is when we're querying things such as
"ensure get X works when database is empty". This is such a low
percentage of tests that it's easier for us to just use
notify_db_session every time, and ensure that all our tests run much
more consistently, at the cost of a small bit of performance when
running tests.

We used to use notify_db to access the session object for manually
adding, committing, etc. To dissuade usage of that fixture I've moved
that to the `notify_db_session`. I've then removed all uses of notify_db
that I could find in the codebase.

As a note, if you're writing a test that uses a `sample_x` fixture, all
of those fixtures rely on notify_db_session so you'll get the teardown
functionality for free. If you're just calling eg `create_x` db.py
functions, then you'll need to make you add notify_db_session fixture to
your test, even if you aren't manually accessing the session.

tests/app/dao/test_service_inbound_api_dao.py

@@ -45,7 +45,7 @@ def test_save_service_inbound_api(sample_service):
     assert versioned.version == 1
 
 
-def test_save_service_inbound_api_fails_if_service_does_not_exist(notify_db, notify_db_session):
+def test_save_service_inbound_api_fails_if_service_does_not_exist(notify_db_session):
     service_inbound_api = ServiceInboundApi(
         service_id=uuid.uuid4(),
         url="https://some_service/inbound_messages",