create webauthn 2fa page

if user has `webauthn_auth` as their auth type, then redirect them to an
interstitial that prompts them to click on a button which right now just
logs to the JS console, but in a future commit will open up the webauthn
browser prompt

content is unsurprisingly not final.

tests/app/test_navigation.py

@@ -293,6 +293,7 @@ EXCLUDED_ENDPOINTS = tuple(map(Navigation.get_endpoint_with_blueprint, {
     'two_factor_email',
     'two_factor_email_interstitial',
     'two_factor_email_sent',
+    'two_factor_webauthn',
     'update_email_branding',
     'update_letter_branding',
     'upload_a_letter',