DRY-up email revalidation check

Previously this was duplicated between the "two_factor" and the
"webauthn" views, and required more test setup. This DRYs up the
check and tests it once, using mocks to simplify the view tests.

As part of DRYing up the check into a util module, I've also moved
the "is_less_than_days_ago" function it uses.

tests/app/utils/test_login.py

@@ -0,0 +1,19 @@
+import pytest
+from freezegun import freeze_time
+
+from app.models.user import User
+from app.utils.login import email_needs_revalidating
+
+
+@freeze_time('2020-11-27T12:00:00')
+@pytest.mark.parametrize(('email_access_validated_at', 'expected_result'), (
+    ('2020-10-01T11:35:21.726132Z', False),
+    ('2020-07-23T11:35:21.726132Z', True),
+))
+def test_email_needs_revalidating(
+    api_user_active,
+    email_access_validated_at,
+    expected_result,
+):
+    api_user_active['email_access_validated_at'] = email_access_validated_at
+    assert email_needs_revalidating(User(api_user_active)) == expected_result

tests/app/utils/test_time.py

@@ -0,0 +1,25 @@
+import pytest
+from freezegun import freeze_time
+
+from app.utils.time import get_current_financial_year, is_less_than_days_ago
+
+
+@pytest.mark.parametrize("date_from_db, expected_result", [
+    ('2019-11-17T11:35:21.726132Z', True),
+    ('2019-11-16T11:35:21.726132Z', False),
+    ('2019-11-16T11:35:21+0000', False),
+])
+@freeze_time('2020-02-14T12:00:00')
+def test_is_less_than_days_ago(date_from_db, expected_result):
+    assert is_less_than_days_ago(date_from_db, 90) == expected_result
+
+
+@pytest.mark.parametrize('datetime_string, financial_year', (
+    ('2021-01-01T00:00:00+00:00', 2020),  # Start of 2021
+    ('2021-03-31T22:59:59+00:00', 2020),  # One minute before midnight (BST)
+    ('2021-03-31T23:00:00+00:00', 2021),  # Midnight (BST)
+    ('2021-12-12T12:12:12+01:00', 2021),  # Later in the year
+))
+def test_get_financial_year(datetime_string, financial_year):
+    with freeze_time(datetime_string):
+        assert get_current_financial_year() == financial_year