Demo page

[KiraLT]

Create a read-only demo page (which allows only watching predefined torrents). It would be great if users could easily check how does it look before installing.

Dependencies:

• Ability to disable adding new torrents. It would be useful only if admins (admin will require login with password) could create new torrents.
• Specify default torrents in the config file and show them in UI home page #267

Maybe it would be possible to automatically deploy to Heroku if we disable seeding.