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.

gulpfile.js

@@ -178,6 +178,7 @@ const javascripts = () => {
     paths.src + 'javascripts/templateFolderForm.js',
     paths.src + 'javascripts/collapsibleCheckboxes.js',
     paths.src + 'javascripts/radioSlider.js',
+    paths.src + 'javascripts/registerSecurityKey.js',
     paths.src + 'javascripts/updateStatus.js',
     paths.src + 'javascripts/homepage.js',
     paths.src + 'javascripts/main.js',