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/main/views/user_profile.py

@@ -23,7 +23,11 @@ from app.main.forms import (
     TwoFactorForm,
 )
 from app.models.user import User
-from app.utils import user_is_gov_user, user_is_logged_in
+from app.utils import (
+    user_is_gov_user,
+    user_is_logged_in,
+    user_is_platform_admin,
+)
 
 NEW_EMAIL = 'new-email'
 NEW_MOBILE = 'new-mob'
@@ -225,3 +229,11 @@ def user_profile_disable_platform_admin_view():
         'views/user-profile/disable-platform-admin-view.html',
         form=form
     )
+
+
+@main.route("/user-profile/security-keys", methods=['GET'])
+@user_is_platform_admin
+def user_profile_security_keys():
+    return render_template(
+        'views/user-profile/security-keys.html',
+    )