Extract user utility code into own module

This provides more room for expansion, and reduces the amount of
arbitrary code in the __init__.py file for the new package.

tests/app/main/views/test_manage_users.py

@@ -5,7 +5,7 @@ import pytest
 from flask import url_for
 
 import app
-from app.utils import is_gov_user
+from app.utils.user import is_gov_user
 from tests.conftest import (
     ORGANISATION_ID,
     ORGANISATION_TWO_ID,