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/notifications/test_rest.py

@@ -544,13 +544,13 @@ def test_get_notifications_for_service_returns_merged_template_content(client, s
 
 
 def test_get_notification_selects_correct_template_for_personalisation(client,
-                                                                       notify_db,
+                                                                       notify_db_session,
                                                                        sample_template):
     create_notification(sample_template)
     original_content = sample_template.content
     sample_template.content = '((name))'
     dao_update_template(sample_template)
-    notify_db.session.commit()
+    notify_db_session.commit()
 
     create_notification(sample_template, personalisation={"name": "foo"})