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.

app/utils/time.py

@@ -0,0 +1,20 @@
+from datetime import datetime
+
+import pytz
+from dateutil import parser
+from notifications_utils.timezones import utc_string_to_aware_gmt_datetime
+
+
+def get_current_financial_year():
+    now = utc_string_to_aware_gmt_datetime(
+        datetime.utcnow()
+    )
+    current_month = int(now.strftime('%-m'))
+    current_year = int(now.strftime('%Y'))
+    return current_year if current_month > 3 else current_year - 1
+
+
+def is_less_than_days_ago(date_from_db, number_of_days):
+    return (
+        datetime.utcnow().astimezone(pytz.utc) - parser.parse(date_from_db)
+    ).days < number_of_days