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/login.py

@@ -3,6 +3,7 @@ from functools import wraps
 from flask import redirect, request, session, url_for
 
 from app.models.user import User
+from app.utils.time import is_less_than_days_ago
 
 
 def redirect_to_sign_in(f):
@@ -41,6 +42,10 @@ def redirect_when_logged_in(platform_admin):
     return redirect(url_for('main.show_accounts_or_dashboard'))
 
 
+def email_needs_revalidating(user):
+    return not is_less_than_days_ago(user.email_access_validated_at, 90)
+
+
 # see http://flask.pocoo.org/snippets/62/
 def _is_safe_redirect_url(target):
     from urllib.parse import urljoin, urlparse