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_users_dao.py

@@ -159,7 +159,7 @@ def test_update_user_attribute(client, sample_user, user_attribute, user_value):
 
 
 @freeze_time('2020-01-24T12:00:00')
-def test_update_user_password(notify_api, notify_db, notify_db_session, sample_user):
+def test_update_user_password(notify_api, notify_db_session, sample_user):
     sample_user.password_changed_at = datetime.utcnow() - timedelta(days=1)
     password = 'newpassword'
     assert not sample_user.check_password(password)