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/assets/javascripts/authenticateSecurityKey.js

@@ -0,0 +1,13 @@
+(function (window) {
+    "use strict";
+
+    window.GOVUK.Modules.AuthenticateSecurityKey = function () {
+        this.start = function (component) {
+            $(component)
+                .on('click', function (event) {
+                    event.preventDefault();
+                    console.log('pretend you just logged in okay');
+                });
+        };
+    };
+})(window);