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.

app/main/views/sign_in.py

@@ -50,7 +50,7 @@ def sign_in():
             if user.email_auth:
                 return redirect(url_for('.two_factor_email_sent', next=redirect_url))
             if user.webauthn_auth:
-                raise NotImplementedError('webauthn not supported yet')
+                return redirect(url_for('.two_factor_webauthn', next=redirect_url))
 
         # Vague error message for login in case of user not known, locked, inactive or password not verified
         flash(Markup(

app/main/views/two_factor.py

@@ -82,6 +82,13 @@ def two_factor():
     return render_template('views/two-factor.html', form=form, redirect_url=redirect_url)
 
 
+@main.route('/two-factor-webauthn', methods=['GET'])
+@redirect_to_sign_in
+def two_factor_webauthn():
+    redirect_url = request.args.get('next')
+    return render_template('views/two-factor-webauthn.html', redirect_url=redirect_url)
+
+
 @main.route('/re-validate-email', methods=['GET'])
 def revalidate_email_sent():
     title = 'Email resent' if request.args.get('email_resent') else 'Check your email'