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.

tests/javascripts/registerSecurityKey.test.js

@@ -0,0 +1,26 @@
+beforeAll(() => {
+  require('../../app/assets/javascripts/registerSecurityKey.js');
+})
+
+afterAll(() => {
+  require('./support/teardown.js');
+})
+
+describe('Register security key', () => {
+  beforeEach(() => {
+    document.body.innerHTML = `
+      <a href="#" role="button" draggable="false" class="govuk-button govuk-button--secondary" data-module="register-security-key">
+        Register a key
+      </a>`;
+  })
+
+  test('it is not implemented yet', () => {
+    window.GOVUK.modules.start();
+    jest.spyOn(window, 'alert').mockImplementation(() => {});
+
+    button = document.querySelector('[data-module="register-security-key"]');
+    button.click();
+
+    expect(window.alert).toBeCalledWith('not implemented')
+  })
+})