refactor sample_user to create a new create_user function

this create_user function can be imported for use in creating specific
users in your tests, for example

```
from tests.app.db import create_user
def test_create_user_persists_to_database(notify_db_session):
    user = create_user()
    assert User.query.count() == 1
```

this has the benefit of not requiring you to pass the notify_db and
notify_db_session fixtures around, and separating custom object
creation from the fixture dependency trees to aid clarity

I started with sample_user since it has no downstream dependencies, but
the intention is to push this out to all db fixtures eventually. This
is a total conversion, but can be rolled out in a non-breaking manner
by keeping arguments in the fixture, and passing them through to the
new db function - then tests can be updated to use the create_* instead
of sample_* functions as and when you want to

tests/app/db.py

@@ -0,0 +1,17 @@
+from app.models import User
+from app.dao.users_dao import save_model_user
+
+
+def create_user(mobile_number="+447700900986", email="notify@digital.cabinet-office.gov.uk"):
+    data = {
+        'name': 'Test User',
+        'email_address': email,
+        'password': 'password',
+        'mobile_number': mobile_number,
+        'state': 'active'
+    }
+    usr = User.query.filter_by(email_address=email).first()
+    if not usr:
+        usr = User(**data)
+    save_model_user(usr)
+    return usr