Extend HTML index page with user interface

[nichtich]

The index page should be kept as simple, static HTML but enriched with a user interface in pure JavaScript

• Show which actions require authentication (this can be done in ESM, no JavaScript)
• Show number of items
• Allow to search & browse items
• Allow to post JSON data via online form
  • at least if no authentication is required
  • with authentication
• Allow to perform additional admin tasks (Admin interface #39)