Add page for security keys with stubbed data

This adds a new platform admin settings row, leading a page which
shows any existing keys and allows a new one to be registered. Until
the APIs for this are implemented, the user API client just returns
some stubbed data for manual testing.

This also includes a basic JavaScript module to do the main work of
registering a new authenticator, to be implemented in the next commits.

Some more minor notes:

- Setting the headings in the mapping_table is necessary to get the
horizontal rule along the top (to match the design).

- Setting caption to False in the mapping_table is necessary to stop
an extra margin appearing at the top.

app/assets/javascripts/registerSecurityKey.js

@@ -0,0 +1,14 @@
+(function(window) {
+  "use strict";
+
+  window.GOVUK.Modules.RegisterSecurityKey = function() {
+    this.start = function(component) {
+
+      $(component)
+        .on('click', function(event) {
+          event.preventDefault();
+          alert('not implemented');
+        });
+    };
+  };
+})(window);