Move ZooKeeperLauncher.config initialization code into one block.
Fix JavaScript style errors.
Refactor admin page templates.

- admin_common: a common template for pages without header menu.
- admin_main: a common template for pages with header menu. It internally uses
  admin_common template.
Add AuthenticatedAction.

AuthenticatedAction redirects to the login page if the user is not logged in.
Add admin user list page.

This page shows all admin users stored in the database.