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/conftest.py

@@ -69,6 +69,10 @@ def create_test_db(database_uri):
 
 @pytest.fixture(scope='session')
 def notify_db(notify_api, worker_id):
+    """
+    Manages the connection to the database. Generally this shouldn't be used, instead you should use the
+    `notify_db_session` fixture which also cleans up any data you've got left over after your test run.
+    """
     assert 'test_notification_api' in db.engine.url.database, 'dont run tests against main db'
 
     # create a database for this worker thread -
@@ -103,7 +107,13 @@ def sms_providers(notify_db):
 
 @pytest.fixture(scope='function')
 def notify_db_session(notify_db, sms_providers):
-    yield notify_db
+    """
+    This fixture clears down all non static data after your test run. It yields the sqlalchemy session variable
+    so you can manually add, commit, etc if needed.
+
+    `notify_db_session.commit()`
+    """
+    yield notify_db.session
 
     notify_db.session.remove()
     for tbl in reversed(notify_db.metadata.sorted_tables):